High Efficiency Tandem Solar Cells and A Method for Fabricating Same

ABSTRACT

Solar cell structures comprising a plurality of solar cells, wherein each solar cell is separated from adjacent solar cell via a tunnel junction and/or a resonant tunneling structure (RTS), are described. Solar cells are implemented on Ge, Si, GaN, sapphire, and glass substrates. Each of the plurality of solar cells is at least partially constructed from a cell material which harnesses photons having energies in a predetermined energy range. In one embodiment each solar cell comprises of at least two sub-cells. It also describes a nano-patterned region/layer to implement high efficiency tandem/multi-junction solar cells that reduces dislocation density due to mismatch in lattice constants in the case of single crystalline and/or polycrystalline solar cells. Finally, solar structure could be used as light-emitting diodes when biased in forward biasing mode. The mode of operation could be determined by a programmed microprocessor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part application, and claimspriority to and benefit of the filing date, of Co-Pending U.S. patentapplication Ser. No. 12/803,946 filed Jul. 9, 2010 (which claimspriority to U.S. Provisional Patent Application No. 61/270,489, filedJul. 9, 2009), the contents of both of which are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

This invention relates generally to solar cells and more particularly totandem solar cells, and a method for fabricating the tandem solar cells,whereby the solar cells efficiently harness solar energy in an energyrange that conventional tandem solar cells cannot efficiently harnessenergy.

BACKGROUND OF THE INVENTION

Tandem solar cells that incorporate semiconductors having various energybandgaps using one or more p-n homojunctions and/or heterojunctions aregenerally integrated via p+-n+ tunnel junctions. FIG. 1 illustrates atandem solar cell structure and shows the integration of two (2) n-phomojunction cells via one (1) p+-n+ GaAs tunnel junction [1]. As shown,the tandem solar cell includes a top cell 12 and a bottom cell 13separated by a tunnel cell 19, where the bottom cell is made of n-p GaAsand the top cell is realized using n-p GaInP (Eg=1.86 eV). As is known,tandem cells efficiently harness solar energy and reduce excess energylosses by absorbing different energy photons in semiconductor layershaving different energy gaps. For example, a two-junction cell using aGaAs substrate and operating at ˜25.7% efficiency, as shown herein,harnesses photon energies above 1.86 eV in the top cell 102 and photonenergies in the range of 1.86 eV-1.42 eV in the bottom cell 104. In someinstances, cells [2] having 37.9% efficiency under 10× sunlightconcentration have been reported. Moreover, referring to FIG. 2a , sometandem cell structures realized on Ge substrates (see FIG. 2a formaterials) can potentially produce efficiencies of up to 63.1% [3,4].

Unfortunately, however, as can be seen from the solar spectrumillustrated in FIG. 2b , these multi-junction III-V cells (i.e. cellsthat are at least partially constructed from Group III-V semiconductormaterials) do not efficiently harness photons having energies above 2.2eV. Generally, the cells described in References 1-5 are GaAs and/or Gebased multi-junction structures that may include GaInP, GaInAs andGaInNAs layers with tunnel junctions. For example, one factor thatlimits the attainment of a predicted efficiency in triple junction III-Vcells is the adverse affect of band offset voltage based losses [5].Table I immediately below shows the offset voltage in multi-junctionsolar cells [5].

TABLE I Offset Voltage (based loss) in Multi-junction III-V Solar CellsBand gap Offset Volt Semiconductor (eV) [(E_(g)/e)—V_(oc)] GaAs 1.44 eV0.387 V Ga_(0.83)In_(0.17)As 1.118 eV 0.390 V GaInNAs 1.07 eV 0.600 VMoreover, using Ge and GaAs substrates is relatively expensive and thesematerials are not abundant in nature.

As such, a method for growing tandem cells is needed whereby Sisubstrates (single crystal and/or polycrystalline) and Si-on-glass orSi-on-quartz or Si-on-sapphire are used for the growth of Ge epitaxiallayers (with reduced dislocation densities resulting from latticemismatch), wherein the tandem cells efficiently harness photons havingenergies above 2.2 eV.

Additionally, tandem solar cells use various energy bandgapsemiconductors, incorporating one or more p-n homojunctions and/orheterojunctions, are generally integrated on a compatible substrate viap+-n+ tunnel junctions. FIG. 1 illustrates a tandem solar cell structureand shows the integration of two (2) n-p homojunction cells via one (1)p+-n+ GaAs tunnel junction. As shown, the tandem solar cell includes atop cell 12 and a bottom cell 13 separated by a tunnel cell 19, wherethe bottom cell is made of n-p GaAs and the top cell is realized usingn-p GaInP (E_(g)=1.86 eV). Tandem cells efficiently harness solar energyand reduce excess energy losses by absorbing different energy photons insemiconductor layers having different energy gaps. For example, atwo-junction cell using a GaAs substrate and operating at ˜25.7%efficiency, as shown herein, harnesses photon energies above 1.86 eV inthe top cell 102 and photon energies in the range of 1.86 eV-1.42 eV inthe bottom cell 104. In some instances, cells having 37.9% efficiencyunder 10× sunlight concentration have been reported. Moreover, sometandem cell structures realized on Ge substrates can potentially produceefficiencies of up to 63.1%. One factor that limits the attainment of apredicted efficiency in triple junction III-V cells is the adverseeffect of band offset voltage-based losses. There are stacked cellsusing amorphous Si layers.

SUMMARY OF THE INVENTION

A solar cell structure is provided and includes a plurality of solarcells, wherein each of the plurality of solar cells is separated fromeach adjacent solar cell via at least one of a tunnel junction or aresonant tunneling structure interface. When the solar cell structureincludes the tunnel junction, the tunnel junction cell includes twohighly doped semiconducting material layers having opposite conductivityto each other, wherein one of the highly conducting layers is of thesame conductivity type as the top layer of the solar cell structure onwhich it is deposited, and wherein the other of the two highlyconducting layers is of the conductivity as that of the bottom layer ofthe solar cell structure on which it is deposited. When the solar cellstructure includes the resonant tunneling structure, the resonanttunneling structure is located between at least one set of two adjacentsolar cells, the resonant tunneling structure including a set of atleast two thin quantum barrier layers and a thin quantum well layer,wherein the resonant tunneling structure is configured to facilitatecurrent flow between semiconducting layers belonging to two differentsolar cells having different energy gaps. Each of the plurality of solarcells is at least partially constructed from a semiconductor material,wherein the semiconductor material has an energy band gap that harnessesphotons having energies in a predetermined energy range which isresponsive to its energy gap, and wherein each of the plurality of solarcells includes at least one of a p-n junction, an n-p junction, or aSchottky interface, and each of the plurality of solar cells isconfigured to harness energies in a different solar spectral energyrange than the other of the plurality of solar cells.

A method of fabricating high efficiency multiple tandem solar cells on asubstrate whose lattice constant differs from the lattice constant ofthe solar cells is provided and includes patterning of a thin maskinglayer including at least one of a SiO₂, SiON, or Si₃N₄ material grown ordeposited on the substrate, wherein the substrate includes exposed Siregions. The method includes growing epitaxially thin buffer layersfollowed by a transition layer which transistions the lattice constantof the substrate on top of the buffer layers and gliding dislocationsfrom the transition layer into the masking layer walls using heattreatment. Additionally, the method includes performing epitaxial growthof at least one base semiconductor, above the transition layers andperforming lateral epitaxial overgrowth of the base semiconductor overthe thin masking layer and building a lattice-matched tandem solar cellstructure on the base semiconductor layer, wherein the tandem cells havea plurality of cells comprised of multiple cells harnessing energy fromdifferent spectral ranges, wherein the plurality of cells are separatedby at least one of a tunnel junction and a resonant tunneling structure.

A method of fabricating antireflection coatings having a two-dimensionalgrating on a substrate is provide and includes constructing gratings onthe substrate, wherein the gratings are constructed using self-assembledSiOx-cladded Si and GeOx-cladded Ge nanodots, wherein the nanodotsinclude an index of refraction, a cladding and a core thickness, andadjusting the index of refraction of the nanodots by adjusting thethicknesses of the nanodot cladding and nanodot core thickness.

This invention relates to solar cell structures comprising a pluralityof solar cells, wherein each of the plurality of solar cells isseparated from each adjacent solar cell via a tunnel cell and/or aresonant tunneling structure (RTS). The solar cell structure isfabricated on substrate one selected from single crystalline Si,polycrystalline Si nano-crystalline Si, GaN, GaN on sapphire, GaN onSiC, glass, and transparent conducting oxide coated glass. Each of theplurality of solar cells is at least partially constructed from a cellmaterial which harnesses photons in a predetermined energy range.

In one embodiment, one or more of solar cells comprising the solarstructure have at least two sub-cells separated by a tunnel junction. Italso describes high efficiency multi-junction tandem solar cells on Si,sapphire and other substrates. There are stacked cells using amorphousSi layers and cladded quantum dot layers. In one embodiment of solarcell structure, a nano-patterned region is used to reduce dislocationsdue to lattice constant differences. The solar cell structures can beconfigured as light-emitting diodes by forward biasing.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional schematic block diagram illustrating oneembodiment of a two-junction tandem cell on a GaAs substrateincorporating a tunnel junction.

FIG. 2a is a cross-sectional schematic block diagram illustratingmaterials for multi-junction Ge-compatible solar cells (and theirefficiencies).

FIG. 2b are graphs illustrating the utilization of photons by Si at AM1(left) and the utilization of photons by multi-junction cells (right).

FIG. 3a is a schematic block diagram illustrating one embodiment of aResonant Tunneling Structure (RTS) used in wide energy gap laserstructures.

FIG. 3b is an energy band diagram for an RTS structure designed for P-pheterointerfaces.

FIG. 3c is a graph illustrating electrical characteristiccurrent-voltage (i-v) of a RTS interface (simulated).

FIG. 4a is a cross-sectional schematic block diagram illustrating oneembodiment of a four-junction tandem cell structure which incorporatesone II-VI (cell #4), two III-V cells (Cells #3 and #2), and one Ge cell(Cell #1). Additionally, in this case two RTS structures are shownschematically without details and an II-VI epitaxial layer is shown overthe Ge layer of Cell #1.

FIG. 4b is a cross-sectional schematic block diagram illustrating oneembodiment of a tandem cell structure where n-p heterojunctions are usedin addition to traditional two-homojunction GaAs and GaInP cells. Thisis in contrast to FIG. 4a in which only homojunctions are used.

FIG. 4c is a schematic block diagram illustrating one embodiment of a5-cell tandem structure using heterojunction cells.

FIG. 4d is a schematic block diagram illustrating one embodiment of aRTS structure which is integrated between P—Ge layer and N—GaAs layersof Heterojunction #1 (of FIG. 4c ).

FIG. 4e is a cross-sectional schematic block diagram illustrating oneembodiment of cascaded two RTS structures which are integrated betweenP—Ge layer and N—GaAs layers of Heterojunction #1 (of FIG. 4c ).

FIG. 5 is a cross-sectional schematic block diagram illustrating oneembodiment of four cells integrated in tandem using tunnel junctions aswell as detailed layering comprising resonant tunneling structures,where cell #4 is a heterojunction cell.

FIG. 6a is a cross-sectional schematic block diagram illustrating oneembodiment of 4 tandem cells with cell #4 as a homojunction (i.e.ZnSeTe—ZnSeTe) and/or n-ZnCdSe/p-ZnSeTe cell without band offsets.

FIG. 6b is a cross-sectional schematic block diagram illustrating oneembodiment of five solar cells in tandem using RTS interfaces and tunneljunctions.

FIG. 7a is a cross-sectional schematic block diagram illustrating across-section of one embodiment of a single-junction quantum dot basedsolar cell structure realized on a quartz substrate using self-assembledquantum dots in a ZnSe matrix.

FIG. 7b is a cross-sectional schematic block diagram illustrating oneembodiment of a back-illuminated tandem two cell structure realized on aquartz substrate using self-assembled quantum dots (three dot sizes withreducing energy gaps) harnessing three spectral ranges (no RTSinterfaces are shown here).

FIG. 8 is a cross-sectional schematic block diagram illustrating oneembodiment of a front illuminated tandem cell comprising a Gehomojunction bottom cell and a top quantum dot cell having at least twosizes of nanodots. The substrate may be single crystal germanium. Here,although no RTS interface is shown, at least one may be utilized.

FIG. 9a is a schematic block diagram illustrating a cross-section of oneembodiment of a two junction tandem cell comprising Ge quantum dots inZnSe based matrix realized on a Silicon-on-sapphire substrate. Si onglass or other substrates may also be used.

FIG. 9b is a schematic block diagram illustrating one embodiment of aRTS structure incorporated under the first set of quantum dots.

FIG. 10 is a schematic block diagram illustrating one embodiment of aback-illuminated Si quantum dots in ZnS matrix in a single cellconfiguration on Si-on-sapphire substrate.

FIG. 11 is a schematic block diagram illustrating a cross-section of oneembodiment of a front illuminated cell having quantum dot on Sisubstrates. Polycrystalline Si substrates are also envisioned and may beused.

FIG. 12A is a schematic block diagram illustrating one embodiment of aLateral epitaxial overgrowth of Ge epi layer on Si substrates usingII-VI buffer layers. Ge layer may be used as a starting point for tandemcells as shown and described herein.

FIG. 12B is a schematic block diagram illustrating one embodiment ofSiO_(x)—Si quantum dots self-assembled in smaller islands (than ispossible by photolithography using site-specific self-assembly).

FIG. 13 is a schematic block diagram illustrating one embodiment of atandem cell with epitaxial p-n junction and self-organized quantum dotforming the absorption region. The cells are shown interfaced using RTSand tunnel junction cells.

FIG. 14a is a diagram illustrating one embodiment of a patternednanostructure grating serving as antireflection coatings for a broaderspectral range using self-assembled cladded nanodots.

FIG. 14b shows a prior art two-dimensional grating.

FIG. 14c shows a structure where uniform layers can be grown withvariable index of refraction which is realized by the assembly ofGeO_(x)-cladded Ge nanodots and SiOx-cladded Ge nanodots.

FIG. 15a shows the layers of InGaAs grown on Si substrate integratingdislocation reduction methodology as shown in FIG. 12.

FIG. 15b shows integrated InGaAs-FETs (with quantum dot gates) on Sisubstrate integrating dislocation reduction techniques.

FIG. 16 Tandem cells using Si (bottom cell, homojunction), RTSstructure, N—GaP-p(GaAsP—GaP) multiple quantum well heterojunction cell.

FIG. 17a shows homojunction and heterojunction cells integrated with RTSand tunnel junction interfaces.

FIG. 17b shows a structure where a RTS is incorporated at theheterojunction (nGaAs-pGe).

FIG. 18a shows a Si cell integrated with dislocation reduction structureproviding interface for III-V and II-VI tandem cells.

FIG. 18b shows a Si cell integrated with dislocation reductionmethodology integrating structure consisting of SiOx-Si cladded dotsproviding interface for III-V and II-VI tandem cells.

FIG. 19 shows a cross-sectional schematic block diagram illustrating oneembodiment of a two-junction tandem cell on a GaAs substrateincorporating a tunnel junction, in accordance with the prior art.

FIG. 20 shows a solar cell structure, comprising of two solar cells, afirst solar cell constructed from Si substrate (such as singlecrystalline, polycrystalline, and nano-crystalline) and the second solarcell constructed from p-CdTe and n-CdS sandwiching a tunnel junction anda resonant tunneling structure, in accordance with one embodiment.

FIG. 21A shows a solar cell structure, comprising of two solar cells, afirst solar cell constructed from nCdS-pCdTe on a glass substrate withtransparent conducting oxide and a second solar cell fabricated usingSi, in accordance with one embodiment. The two cells are interfaced witha resonant tunneling structure and a tunnel junction.

FIG. 21B shows a solar cell structure which comprises two solar cells,in accordance with one embodiment. It is similar to the structure ofFIG. 21A in terms of first solar cell. The difference is in thestructure of second solar cell. The second solar cell comprises a firstn-type layer and a second p-type layer. The first n-type layer is oneselected from n-type amorphous Si, n-type amorphous Ge, n-typepolycrystalline Si, and n-type polycrystalline Ge. The second p-typelayer comprises multiple cladded quantum dot layers and one selectedfrom p-type amorphous Si, p-type amorphous Ge, p-type polycrystallineSi, and p-type polycrystalline Ge.

FIG. 22A shows a solar cell structure disposed on a GaN substrate,comprising a plurarity of solar cells, in accordance with oneembodiment. The first cell is constructed from p-GaN layer, GaN—InGaNmultiple quantum well layers, and an n-type GaN layer. Second solar cellis constructed a first p-type layer and a second n-type layer, and bothfirst and second layers are constructed of AlGaInP. The third solar cellis made of GaInP, and the fourth cell is made of semiconductor oneselected from GaAs and GaInAs. The GaN based first solar cell isinterfaced via a nanopatterned region that facilitates growth of singlecrystalline GaAs and compatible semiconductors such as AlGaInP, GaInP,GaAs and GaInAs.

FIG. 22B shows a solar cell structure similar to FIG. 22A with theprimary difference being in the substrate used is sapphire, inaccordance with one embodiment. In addition, the top Ohmic contact gridis formed on p-GaN layer disposed on the sapphire substrate. The bottomOhmic contact is disposed on the n+-type facilitator layer deposited onthe second n-type layer of the fourth solar cell.

FIG. 22C shows a solar cell structure similar to that of FIG. 22A andFIG. 22B with the primary difference being in the construction of firstp-type layer and second n-type layer of one or more plurality of solarcells, in accordance with one embodiment. In FIG. 22A and FIG. 22B thesecond solar cell 3017 is shown to comprise of two layers; first p-typelayer 3018 and second p-type layer 3019. Both of these layers are shownto be made of AlGaInP. However, in the solar cell structure of FIG. 22Cthe second solar cell is shown with first p-type layer comprising ofthree sub-layers—first sub-layer, second sublayer, and third sub-layer.Similarly, the second n-type layer comprises of three sub-layers firstsub-layer, second sub-layer, and third sub-layer. The energy gaps ofsub-layers may be different from each other.

FIG. 22D shows a solar cell structure comprising a plurality of solarcells including first solar cell, a second solar cell, a third solarcell, a fourth solar cell and a fifth solar cell, in accordance with oneembodiment. The first solar cell comprises a first p-type layer and asecond n-type layer. In one embodiment, the first p-type layer serves asthe silicon substrate. An n-type layer is disposed to form an n-phomojunction making the first solar cell. The second cell is comprisedof GaAs or GaInAs, the third cell is made of GaInP, the fourth cell ismade of AlGaInP, and the fifth solar cell is constructed from ZnSe andrelated semiconductors. The solar cell structure comprisesnano-patterned regions interfacing first Si-based solar cell withGaAs-lattice matched solar cells.

FIG. 23A shows a solar cell structure comprising a plurality of solarcells disposed on p-Ge substrate, in accordance with one embodiment. Inone embodiment, Ge serves as the substrate for four other solar cells.In another embodiment, the p-type Ge substrate forms the first solarcell on which resonant tunneling structures, tunnel junctions and fourother solar cells are disposed. The five-cell structure comprises firstsolar cell constructed of p-type Ge substrate and an n-type Ge layer.The second solar cell is constructed from GaAs, the third solar cell isconstructed of GaInP, the fourth solar cell is constructed from AlGaInP,and the fifth solar cell is constructed from ZnSe. Here, some solarcells (e.g. second solar cell) are comprised of two solar sub-cells, abottom sub-cell and a top sub-cell. A tunnel junction separates thebottom sub-cell and top sub-cell.

FIG. 23B shows a GaAs solar cell comprising of a bottom solar sub-cell,tunnel junction and a top solar sub-cell fabricated on an un-doped GaAssubstrate, in accordance with one embodiment.

FIG. 23C shows a voltage-current (V-I) plot of a GaAs solar cell withtwo sub-cells, in accordance with one embodiment. The current I in mA isthe vertical axis and voltage V in Volts is the horizontal axis. Theillumination was done using a white light lamp.

FIG. 24 shows a schematic block diagram showing four solar cellstructure interfaced to a switching block controlled by amicroprocessor, in accordance with one embodiment. The bidirectionalconverter enables powering of a load (or battery) using sunlight orconnecting solar cell structure in a mode when it serves as a lightemitting device, in accordance with one embodiment.

DETAILED DESCRIPTION

In accordance with the present invention, tandem solar cells and amethod for fabricating the tandem solar cells is provided, whereby thesolar cells efficiently harness solar energy in an energy range thatconventional epitaxial Ge/GaAs/GaInP based tandem solar cells cannotefficiently harness energy. Additionally, a methodology to fabricatehigh efficiency solar cells on Si substrates using novel dislocationreduction technique is also provided.

In accordance with the invention, a solar cell is provided thatincorporates wider energy gap materials to harness solar energy in the2.2 eV to 3.7 eV range. This is advantageous because conventionalepitaxial Ge/GaAs/GaInP based tandem cells do not efficiently convertenergies from photons having energies in this range. As is shown anddiscussed herein below, the integration of multi-junction cells thatharness excess energy loss in the range of 2.2 eV-3.7 eV solarradiation, using Ge/GaAs compatible II-VI semiconductors such as, forexample, ZnSSe, ZnSTe, and ZnMgSSe is described. In one embodiment, theincorporation of a ZnSSe—AlGaInP heterojunction cell is presented wherethe cell improves conversion efficiencies by harnessing 2.7 eV-2.2 eVphotons. In another embodiment, a cell that incorporates multiple II-VIand II-VI/III-V heterojunctions grown over III-V structures ispresented, where the cell harnesses 3.5 eV-2.7 eV, 2.7 eV-2.2 eV and2.2-1.8 eV photons. In yet another embodiment, tandem solar cells whichincorporate resonant tunneling structures (RTS), are described. Thesestructures reduce the offset voltage at wide energy gap N/low energy gapn or P-p type hetero-interfaces.

In still yet another embodiment, a method to form tandem cells, usingsemiconductor layers having different lattice constants, on a Sisubstrate, with minimized dislocation density resulting in superiorefficiency, is described. This method involves growth of low dislocationdensity epitaxial films using a novel combination of techniquesincluding: (1) nano-patterning of semiconductor layer (on whichheteroepitaxial growth is desired) using SiO₂ mask, (2) growth oflattice mismatched heteroepitaxial layers in these oxide surroundednanoislands, (3) patterned hetero-epitaxial processing (PHP) which mayinclude heat treatment of patterned heteroepitaxial regions to glide andbury the dislocations into oxide walls (see references [6 and 7] for PHPwithout oxide walls and nanoislands), and (4) lateral epitaxialovergrowth techniques [8]. Alternatively, site-specific self-assembly ofnano-dots may be used to create nano-islands which are smaller thanfeatures obtained by lithographic techniques. Accordingly, tandem III-Vand II-VI cells can be grown on Ge layers, which can be grown on moreabundant and relatively inexpensive Si substrates.

Also disclosed are tandem solar cells that use at least one layer ofself-assembled quantum dots (such as cladded Ge and Si dots) on nano-and micro-crystalline semiconducting films, which may be grown oninexpensive substrates, such as Si, glass, and/or quartz. One embodimentof quantum dot based cells involves using variable sized dots tosimulate different energy gaps and to harness selected solar spectrumenergies to minimize excess (hν-E_(g)) energy loss. A novel structurewith epitaxial layers grown in narrow boxes (simulating quantum dots ofsimilar sizes) is also disclosed. In this context, a novel method whichuses a combination of nano-patterning and patterned heteroepitaxialprocessing (PHP) is described to achieve dislocation density reduction.Accordingly, dislocation free nanopatternd regions now can serve tonucleate high quality epitaxial layers (e.g. Ge) using lateral epitaxialovergrowth (over oxide or some other material separating thenano-islands].

Accordingly, in light of the above and as disclosed herein, theinvention includes,

(1) Integration of cells harnessing above 2.2 eV photons to efficientlyincorporate cells that have material that absorbs photons with energiesabove 2.2 eV, including, but not limited to, II-VI wide energy gapsemiconductors in addition to conventional III-V cells along with Gestructures,(2) Use of resonant tunneling structure (RTS) [9] interfaces to reduceor eliminate the effect of band offset related voltage drops athetero-interfaces, which result in reduced conversion efficiency.(3) Use of Resonant Tunneling Structures (RTS) [9] and graded gap [10]techniques have been used in wide energy gap semiconductor lasers, suchas ZnMgSSe—ZnCdSe blue green lasers, to form Ohmic contacts. Asdescribed herein, RTS structures (asymmetric or symmetric) are used toreduce the voltage drop due to band offsets at hetero-interfaces. FIG.3a illustrates a typical RTS and FIG. 3b illustrates its current-voltage(I-V) characteristic, respectively. It should be appreciated that RTSstructures as disclosed herein having homojunction and/or heterojunctiontype cells. Additionally, the application of cascaded RTS structures isalso disclosed. It should also be appreciated that RTS interfaces alongwith tunnel junctions, as disclosed herein, provide a methodology toharness conventional as well as quantum dot based cells by configuringthem as tandem cells with overall higher conversion efficiency.(4) Methodology(s) to produce high efficiency cells built on Silicon(single and/or polycrystalline and/or microcrystalline and/ornanocrystalline) substrates using novel dislocation reductiontechniques; and(5) Quantum dot (QD) structures that absorb in different spectralregimes and which can be configured in tandem cell structures.It should be appreciated that nanowire and quantum dot [11-14], and/orcarbon nanotubes and their combination with quantum dots may also beused to fabricate solar cells.

Referring to FIG. 1, a two-junction tandem cell on GaAs substrate inaccordance with the prior art is shown, where the two-junction cell isoperating at an efficiency of ˜25.7% includes a GaAs cell and an InGaPcell. Here, we have used the layer numbering for Ge (bottom cell) andGaInP (top cell) and tunnel cell or tunnel junction as used in FIG. 4a(which shows the innovation of RTS using this material combination asthe example). The top cell is GaInP homojunction cell (12) and thebottom cell is GaAs homojunction cell (13). The constituent layers forthe top cell are p-GaInP (121) and n-type GaInP (122), and thehomojunction is 27. The constituent layers for the bottom cells arep-GaAs (131) and n-GaAs (132), and the homojunction is 26. The tunneljunction 19 between the two cells is comprised of n+ GaAs (191) and p+GaAs (192) layers. There is another layer of AlInP (240) on top cell,and the patterned contacting cap layer of GaAs (2200) along with Ohmiccontact grid (220) are shown. The antireflection (AR) coating is 230.The substrate is p-GaAs (14100) and the back Ohmic contact is 210. Thetunnel junction layer 192 has an additional GaInP layer 1230. Similarly,there is a p-GaInP layer 1231 in the bottom cell. Note that there is noRTS structure in this cell.

Referring to FIG. 2a , a partial lists of materials for multi-junctionGe-compatible solar cells (and their efficiencies) are illustrated [3,4]. It should be appreciated that in some cases, tandem cells built onGe substrates can potentially produce efficiencies up to 63.1% (usinglayers of GaInP/GaInAs/GaInNAs). It should also be appreciated thattandem cells using metamorphic structure have achieved 40% efficiency.Referring to FIG. 2b , graphs illustrating the utilization of photons bySi (See graph a) and the utilization of photons by multi-junction cells(See graph b) are shown.

Referring to FIG. 3a , FIG. 3b and FIG. 3c , an asymmetric ResonantTunneling Structure (RTS) for wide energy gap lasers is illustrated (SeeFIG. 3a ) along with energy band diagram (See FIG. 3b ) used insimulating electrical characteristic current-voltage (I-V) of the RTSinterface in FIG. 3c ). It should be appreciated that graded gap andother structures could be used to form Ohmic contacts. Here, the RTSfacilitates offset minimized electrical conduction betweenp-AlAs/Al_(x)Ga_(1-x)As layer (1) and p-ZnSe layer (2). RTS comprises aZnCdSe barrier layer (3), quantum well layer (4), and another ZnMgSSe orZnSSe barrier layer (5) with an optional graded layer of ZnCdSe (6). Asshown later in FIG. 4e , a second RTS may be used to replace this gradedlayer. That is, one can use a cascaded (more than one) RTS structures intandem to provide tunnel paths to carriers. The energy levels in thequantum well between two barriers provide a pathway for carriers fromp-AlGaAs layer to tunnel to the p-ZnSe layer without being adverselyaffected by the band offset (ΔE_(v)) manifested at the pAlGaAs-pZnSeinterface.

FIG. 4a shows a four-junction tandem cell structure, which incorporatesa cell (11) (cell #4) constructed from Group II-VI materials (harnessingabove 1.88 eV photons), two cells cells #2 (13) and #3 (12)] where eachare constructed from Group III-V materials (harnessing 1.86 eV-1.4 eVphotons), and a Ge cell #1 (14) (harnessing 1.42-0.67 eV photons). It iscontemplated that a 5-junction cell which optimizes the radiationbetween 1.88-3.7 eV by using two II-VI solar cells constructed fromGroup II-VI materials may also be fabricated. The cell #5 may harnessphotons having energies in the 2.67-3.7 eV range using a ZnSSe—ZnMgSSecell. It is contemplated that a novel Resonant Tunneling Structure (RTS)15 may be incorporated, which is designed to reduce the offset voltagebetween Group III-V material cells and Group II-VI material cells.Additionally, interface between cell #3 (12) [in FIG. 4a which harnessesphotons having energies in the range 1.86 eV-1.42 eV] and cell #2 (13)may be optimized by reducing the offset voltage by the introduction of asuitable RTS (16). Similarly, a RTS structure (17) may be introducedbetween cell 1 (14) and cell 2 (13). FIG. 4a also shows conventionaltunnel junctions. Tunnel junction 18 is between cell #1 (14) and cell #2(13). It consists of an n⁺ Ge layer 181 and a p⁺ Ge layer 182. Cell 1 isformed on a p-Ge substrate 141 and with an n-Ge layer 142 serving as itsthin window region. Cell 2 is formed with p-GaAs [photon absorber] layer131 and n-GaAs layer 132 [serving as its window region]. Tunnel junction#2 (19) consists of n+ GaAs (191) and p+ GaAs (192) layer is formedbetween cell #2 (13) and cell #3 (12). Cell #3 (12) is realized usingp-GaInP (121) and n-GaInP (122). Similarly, tunnel junction #3 (20) iscomprised of an n⁺ layer 201 and a p⁺ layer 202, and is formed betweencell #3 (12) and cell 4 (11). These layers (201 and 202) are shown to bemade of GaAs, but they can be realized in GaInP to avoid band offsetissue. Cell 4 (11) consist of a p-type wide energy gap material such asZnSTe (111) and an n-type layer ZnSe (112). The cell interfaces whereband offset is not an issue, a tunnel junction may be employed withoutan RTS structure. In some cases, wider gap layers such as 143, 133 areincorporated in a homojunctions to improve carrier transport. The bottommost cell #1 (14) has an Ohmic contact 21 and the topmost cell #4 (11)has an appropriate Ohmic contact 22. The Ohmic contact 22 is formed on ahighly doped n+ layer 24. The antireflection coating is shown as 23. Thenovelty here is the incorporation of a resonant tunneling structure(RTS) interface between two cells with and without a tunnel junctionbetween them. For example the RTS 17 is between layer 182 (p⁺Ge) [lowerenergy gap] and layer 131 (p-GaAs) [higher energy gap] as FIG. 3b showsbetween p-AlGaAs [lower energy gap] and p-ZnSe [higher energy gap].Every RTS needs simulation and experimentation to achieve the currentpeak at a reduced voltage drop. Examples of RTS are shown later in FIGS.4d and 4e . RTS comprises at least two quantum barrier and one quantumwell layers.

Table II below illustrates a comparison of the conversion efficienciesof cells constructed from Group III-V materials wherein the cells havesingle, double and triple junctions. In one embodiment, the cell having3-junctions may include two Group III-V material cells and one GroupII-VI material cell (as shown in FIG. 4a , but without the Ge cell). Inanother embodiment, the 4-junction cell may be identically the same asthat shown in FIG. 4. In still yet another embodiment, a 5-junctiontandem cell (not shown) may be fabricated to reduce the excess photonenergy loss in the visible spectral regime (2.2-2.7 eV), and/or toextend the spectral range to about 3.5 eV. It is contemplated that the5-junction tandem cell may include Group II-VI material cells that maybe divided into multiple (such as, for example two (2)) tandem cells toachieve the above and/or to further reduce the excess photon energy lossin the visible spectral regime (2.2 eV-2.7 eV), and/or to further extendthe spectral range to about 3.5 eV.

TABLE II Comparison of Efficiency for various III-V and Proposed4-Junction Cell Single Junction *Double Junction *Triple JunctionII-VI/III-V/Ge Efficiency [Sun Concentration (X)] 3 Junction 4-Junction25% 29% 31% 1   37.8% 57% 31% 39% 40    ~63% 5-Junction/RTSTale II compares various conventional tandem cells [3]. Here, a fivejunction cell (shown in FIG. 6b ) consisting of novel RTS interfaces arealso listed.

Referring to FIG. 4(b), a tandem cell structure is illustrated where twon-p heterojunctions (28 and 30) are used in addition to traditional twohomojunctions (26 and 27) for GaAs and GaInP cells. This is in contrastto FIG. 4a in which only homojunctions are used. Bottom most cell is aGe heterojunction cell consisting of a Ge substrate 141, p-Ge bufferlayer 1410 (optional), forming n-GaAs-pGe heterojunction (28) withn-GaAs window region. The other heterojunction in this structure is 30formed between p-ZnSe layer 31 and n-ZnSSe or n-ZnMgSSe layer 32. It hastwo homojunction cells like FIG. 4a . These are 26 and 27, respectively.Homojunction 26 is formed using p-GaAs (131) and n-GaAs (132), andhomojunction 27 is formed using p-GaInP (121) and n-GaInP (122). FIG. 4bdoes not show the use of RTS. It has three (19, 20, 33) n+-p+GaAs tunneljunctions. The constituent layers of tunnel junctions are shown as 191and 192, 201 and 202, and 331 and 332. In addition, buffer layers 240,123, 34 and 35 are also shown. The top cap layer facilitating the Ohmiccontact to layer 32 is shown as 22 which includes an Ohmic contact. Thebottom Ohmic contact to Ge is shown as 21.

Referring to FIG. 4c , a 5-cell tandem structure is illustrated usingall heterojunction cells. From bottom going to the top antireflectioncoating, the first heterojunction 28 is formed between p-Ge substrate(141 or 141 and 1410 if a buffer is included as shown in FIG. 4b ) andN—GaAs (29). The second heterojunction is 34 which is formed betweenp-GaAs (131) and N—GaInP (133). Each heterojunction and the layersforming it constitute a solar cell targeting a specific part of thesolar spectrum. Sandwiched between two solar cells is tunnel junction #1(19) which is formed by n+GaAs (191) and p+GaAs (192). The thirdheterojunction 35 is formed between p-GaInP (121) and a higher band gaplayer such as n-AlGaInP (42). Tunnel junction #2 interfaces cell 2 andcell 3 and consists of n+GaInP (40) and p+ GaInP (41) layers. The fourthheterojunction 36 is formed between p-AlGaInP layer 45 and n-ZnSe layer46. The tunnel junction #3 (38) consists of an n+AlGaInP layer (43) anda p+ AlGaInP layer (44). Heterojunction #4 (36) comprises of a p-AlGaInPlayer (45) and an n-ZnSe layer (46). An n+ZnSe layer 47 and p+ZnSe layer48 form the tunnel junction #4 (39). Tunnel junction #4 interfaces thecell 4 and cell 5. Cell 5 is comprised of heterojunction #5 (30) and itsconstituent layers p-ZnSe (40) and n-ZnMgSSe layer (50). Finally, thereis an n+ZnSe thin cap layer (51) which facilitates formation of an Ohmiccontact grid (52). The antireflection (AR) coating 53 is also shown.Similarly, a bottom Ohmic contact 21 is also shown.

Referring to FIG. 4d , a RTS structure which is integrated between thep-Ge substrate (141) and N—GaAs layer (29) forming the heterojunction #1(28) (of FIG. 4c ) is illustrated. The RTS is needed to provideconduction of both electrons and holes adequately for the functioning ofthe overall structure. The RTS consists of at least three layers (54,55, and 56). Here, 54 and 56 serves as the quantum barrier layers and 55as the quantum well layer. These layers are thin (30-80 Angstroms) andof energy gaps that result in good carrier transport (such as shown inFIG. 3). These layers should be compatible in terms of lattice constantsand related strains and band gaps. In addition, their doping levels maybe adjusted to provide appropriate band offsets or discontinuities inthe conduction and valence bands.

Moreover, referring to FIG. 4e , a heterostructure is illustrated wheretwo RTS structures are integrated between p-Ge substrate (141) [or p-Gecombination of substrate 141 and other layers (1410 and 1411)] andN—GaAs layer (29) forming a heterojunction (such as shown in FIG. 4c ).Here, we have barrier #B1 54, well #1 55, barrier #B2 56, well #W2 57,barrier #B3 58, well #W3 59, barrier #B4 60. The use of cascadedresonant tunneling structures is stipulated to optimize the carriertransport at the heterointerface. The energy gaps, band offsets, andthickness of various wells and barriers are selected to optimize theflow of both types of carriers adequately.

Referring to FIG. 5, four (4) cells integrated in tandem using tunneljunctions as well as detailed layering comprising resonant tunnelingstructures is illustrated, where cells #1-3 are homojunctions and cell#4 is a heterojunction cell. In this figure the tandem solar cellstructure of FIG. 4a is shown with different types of RTS interfaces.Some RTS (such as 15, 16, and 17 are between p-lower energy gap/p-widerenergy gap semiconductor interfaces involving valence band (VB) andother (such as 61) is between n-lower energy gap/n-wider energy gapinvolving the conduction band (CB) transport. As shown, the four (4)cells are integrated in tandem using tunnel and resonant tunnelingstructures [here 0.1 μm or 0.1 um is equal to 0.1 micron]. It should beappreciated that RTS VB-1 (17) means the Resonant Tunneling Structure(RTS) as shown in detail (as in inset), removes the blocking of currentin the valence band. The RTS may be formed between pGe layer (182) andp-GaAs layer 131 as shown. In this case, the Resonant TunnelingStructure (RTS) has a very thin ZnSe buffer layer 171 which also servesas part of the barrier layer B 1. The buffer layer may be introduced tofacilitate deposition of a Group III-V material layer such as p-GaInP172 (serving also as the part of barrier layer B1) over Ge. However,alternative methods resistant to forming antiphase domains may also beused. The other layers of RTS 17 are well W1 p-GaAs 173 and barrier B2174 shown as p-GaInP. GaAs layer 173 may be substituted by GaInAsmaterial depending on the current transport design. Details of RTS 61between n-GaAs layer 132 and n+-GaInP (1910) is described next. It iscomprised of barrier layer 62 (AlGaInP), well 63 (n-GaInP) and barrier64 (AlGaInP). The barrier layers may be lightly n-doped if needed.Layers n+GaInP (1910) layer forms the tunnel junction 190 along withp+GaInP layer 1920. In FIG. 4a the tunnel junction 19 was formed betweenn+GaAs (191) and p+GaAs (192). To distinguish with GaInP layers we haveused 190 (for GaInP based tunnel junction), 1910 and 1920 numbering. RTS#16 may be needed if the tunnel junction 2 is formed using GaAs layers(191 and 192 as shown in FIG. 4a ). It is not needed the way tunneljunction is shown in FIG. 5a . FIG. 5a shows the incorporation of bothGaInP tunnel junction 190 and GaAs tunnel junction 20 (between cell #3and cell #4) comprising of n+GaAs (201) and p+GaAs (202). Tunneljunction #3 is incorporated between n-GaInP layer 122 and RTS #15. RTS#15 is comprised of a barrier layer 65 (B1 such as p-ZnMgSTe), a well W1layer p-ZnSTe (66), and a barrier B2 layer p-ZnMgSTe (67). It should benoted that each RTS structure has two quantum barriers and one quantumwell layers (cascaded RTS has more). However, their composition isdifferent as identified by various layer numberings. RTS #15 and tunneljunction 20 facilitates transport between Cell #3 and Cell #4. Cell #4is a heterojunction between pZnSTe layer 111 and n-ZnSe layer 112. Thecomposition of ZnSTe (112) and GaInP (121), which determine theirbandgaps, is determined by the optimization of the series current intheir respective cells. Overall, optimum power output is obtained bycombining all 4 cells. The cap layer n-ZnSe (24), top Ohmic contact 22,antireflection coating (23), and bottom/back Ohmic contact 21 are alsoshown.

In another embodiment, use of GaAs tunnel junctions as an alternate toGaInP tunnel junction (used in FIG. 5) is envisioned along withassociated RTS structures. In addition, the top most cell could berealized using Schottky barrier interface or MIS (metal-thininsulator-semiconductor) interface.

Referring to FIG. 6a , four tandem cells having integrated RTS andtunnel junctions. The layer numbering is similar to FIG. 5. Unlike FIG.5, all layers [171 barrier, 172 barrier, 173 well and 174 barrier] ofRTS VB-1 (17) are shown integrated in one structure. In addition, inplace of GaInP based tunnel junction #2 (190) a GaAs-based tunneljunction #2 (19) [see FIG. 4a ) is employed, illustrating flexibilityafforded and adjustments commensurate to the technological feasibility.The layers comprising RTS 16 above the tunnel junction #19 outer layerp+GaAs (192) are as follows. Layer 70 (ZnSe) [whose role is tofacilitate growth and serve as a barrier as well] and layer 71 [AlGaInP]serving as the first barrier B1, layer 72 well W1 (GaInP), and layer 73(AlGaInP) barrier B2. This RTS provides current flow between p+GaAs(192) and p-GaInP (121). Similarly, the RTS (150), above the tunneljunction 190 (formed by n+GaInP layer 1910 and p+GaInP layer 1920), isshown in detail. An embodiment of RTS 150 (shown in inset) includeslayer 74 (ZnSe serving as barrier B1), layer 75 well W1 (ZnSTe), andlayer 76 barrier B2 (ZnSe or ZnMgSTe). In this structure, the II-VI cell#4 is realized using layer 111 (p-ZnSTe) and 1120 (n-ZnSTe).Alternately, layer 112 (n-ZnSe) could be used. Bottom/back contact (21),cap layer 24, top Ohmic contact 22, and AR coating 23 are same as beforein FIG. 5 a.

FIG. 6b illustrates five solar cells in tandem using two Group II-VImaterial cells employing RTS interfaces and tunnel junctions forintegration. The structure is similar to FIG. 6a up to cell #4 layer 111(p-ZnSTe). This structure shows the situation when a tunnel junction onn+ZnSTe and p+ZnSTe is not technically possible. In this situation, theusage of a RTS structure is illustrated. (Note ZnSTe can be replaced byZnCdSe with similar band gap, if technology permits). Layer 111 has ontop of it n+ZnCdSe layer (77). This is followed by a RTS structure (78)between n+ZnCdSe layer (77) and n+GaInP layer (82). This RTS structure78 is comprised of a barrier 79 (selected from a list of n-ZnSe,n-ZnSSe, and n-ZnMgSSe), well 80 (ZnCdSe), and another barrier 81(selected from a list of n-ZnSe, n-ZnSSe, and n-ZnMgSSe). The RTS isdesigned to provide low voltage drop across RTS for the operatingcurrent of the entire solar cell structure. Layer 820 (p+GaInP) form thetunnel junction (83). RTS structure (84) in the valence band facilitatescurrent flow from layer 820 to p-ZnSe absorber layer (88) of Cell #5.RTS 84 has again at least 2 barrier layers and one well layer. The firstbarrier layer (85) is p-ZnMgSSe, the well (86) [selected from a list ofp-ZnSe, ZnCdSe, and ZnSTe], and barrier (87) p-ZnMgSSe. Cell #5 isformed between p-ZnSe (89) and the window region n-ZnSe (90). The caplayer is n+ZnSe (24). The front Ohmic contact 22 is formed on layer 24.AR coating is 23, and the back contact is 21.

Referring to FIG. 7a , it shows the cross-sectional schematic of asingle-junction quantum dot based solar cell structure which is realizedon a quartz or glass substrate 91. A layer of p-Ge (92) is grown on thesubstrate. This layer 92 is selected from a list such as amorphous-pGe,polycrystalline of nanocrystalline p-Ge. It has a highly doped regionforming the grid, shown as 920. Light is illuminated through the glasssubstrate. A thin buffer layer, selected from p-ZnSe, p-ZnSeTe, ZnSTe,layer (93) is grown on p-Ge layer. GeOx-cladded Ge quantum dots (94) areself-assembled in a layer (95). GeOx can be desorbed from Ge dots and athin layer (96) of wide gap semiconductor (such as n-ZnSe or undopedZnSe or ZnSSe) is grown. This layer serves as a matrix which hosts Gequantum dot layer (95). This is followed by self assembly of another setof GeOx-cladded Ge dot layer 98 in which quantum dots 99 are of largerdiameter than dots 94. The desorption of GeOx is followed by the growthof another n-ZnSe or undoped ZnSe layer 100. A highly doped cap layer101 is deposited to form the back Ohmic contact 102. FIG. 7b shows atandem two cell structure having a tunnel junction. The structure isrealized on a quartz substrate (91) using self-assembled quantum dots(three dot sizes) with reducing energy gaps harnessing three spectralranges (no RTS is shown here). The structure is similar to FIG. 7a up tolayer 100 (n-ZnSe). An n-GaAs layer (103) is grown at low temperature.This is followed by layer 104 (n+GaAs) and 105 (p+GaAs) which form thetunnel junction. A thin layer of p-GaAs or p-AlGaAs or p-GaInp (106) isdeposited. This layer has another layer of p-ZnSe (107). The twop-layers serve to self-assemble another set of GeOx-cladded Ge dot layer109 having dots 108 which have an effective band gap smaller than theother dots. Again the GeOx cladding is desorbed and a host lightlyn-doped or undoped ZnSe layer (110) is grown. This is followed by layer113 which is n-doped. The layer 1130 of n+ZnSe serves as the cap layer,and an Ohmic contact 1131 is formed. The illumination is from thesubstrate side.

FIG. 8 illustrates a front illuminated tandem cell comprising a Gehomojunction bottom cell and a top quantum dot cell having at least twosizes of nanodots. Various other size nanodots may also be employed.Although the substrate (141) as shown is a single crystal germanium,other substrates may be used suitable to the desired end result. Layer114 is n-Ge forming a solar cell using homojunctions. Layers 181 (n+Ge)and 182 (p+Ge) forming the tunnel junction 115. A p-type interfaciallayer 116 (selected from a wider gap material such as p-ZnSe) isdeposited. This is followed by the self-assembly or growth of a layer 95of cladded GeOx-Ge quantum dots 94. Upon removal of cladding, a layer 96of wider gap material such as undoped or lightly n-doped ZnSe is grownwhich also forms the host matrix surrounding the Ge dots. This isfollowed by self assembly of another set of GeOx-cladded Ge dot layer 98in which quantum dots 99 are of larger diameter than dots 94. The nextstep is desorption of GeOx which is followed by the growth of anothern-ZnSe or undoped ZnSe layer 100. A p-type layer 117 (such as ZnSe orother semiconductors) can now grown forming the window region of theheterojunction. A highly doped p+ layer 118 is deposited to facilitateforming the top Ohmic contact 119. The back contact 120 provides anotherterminal of the tandem solar structure. In this case, this structuredoes not use an RTS structure, although such a structure may beincorporated as desired. Unlike Si, Ge (though an indirect material withE_(g)=0.67 eV) also has a direct gap (0.88 eV), thus absorbing photonsin a very thin layer as compared to Si.

Referring to FIG. 9a , a two junction tandem cell comprising Ge quantumdots in a ZnSe based matrix realized on a Si film 125, in turn grown ona transparent substrate 124. The semiconductor film-substratecombination is selected from a list including silicon-on-sapphire,Si-on-quartz, etc.). A highly doped region 126 serves as the frontcontact in the form of a grid permitting sun light. Layer 125 has eithera RTS (not shown) or a graded layer region. The ZnS (lattice matched top-Si) buffer layer may be graded to p-ZnSTe 127, upon which layer 128(p-ZnSTe graded to p-ZnSe) is formed. Layer 128 has a layer 95 ofself-assembled Ge quantum dots (94) in a ZnSe layer 96 serving as matrixhosting quantum dots (as described in FIGS. 7 and 8). There is a thinlightly doped or undoped buffer layer 97 upon which is self assembledanother layer 98 of quantum dots 99 (different in size than dots labeledas 94). A ZnSe layer 100 serving as matrix hosting the Ge dots 99. Ann-GaAs layer 129 is deposited on layer 100. Layer 129 hosts a tunneljunction 19 formed between n+GaAs layer 191 and p+GaAs layer 192. Ap-GaAs layer 120 serves as the base on which Ge quantum dots 108 areassembled forming a layer 109. This layer (as described before) ishosted in a ZnSe layer 110. Upon this layer a n-ZnSe or ZnSSe layer 113is formed. Layer 113 is deposited with a highly doped cap layer 1130 onwhich the back Ohmic contact 1131 is formed. It should be appreciatedthat these variable-sized dots may be used to represent various bandgaps (E_(g1), E_(g2), E_(g3) as shown in FIGS. 7, and 8) for photonabsorption like tandem cells. Referring to FIG. 9b , an RTS structureincorporated under the first set of quantum dots. The RTS 139 iscomprised of a barrier layer 135 (ZnS), a well layer 136 (ZnSTe), and abarrier layer 137 (p-ZnSe) layers. This RTS is designed to provide alow-voltage drop structure between p-Si layer 125 and layer 138 uponwhich quantum dot layer 95 is assembled. Layer 138 may be selected froma list of semiconductors p-Ge, p-GaAs, p-GaInP, GaInAs). Other layersare similar to FIG. 9 a.

Additionally, referring to FIG. 10, back-illuminated Si quantum dots ina ZnS matrix in a single cell configuration on Si-on-sapphire substrateis illustrated. Here, sapphire substrate 124 hosts an epitaxial Si layer125. p+ doped grid is formed in 125 to serve as the Ohmic contact 126. Alayer 144 serves as a buffer layer. A layer 146 of Si quantum dots 145is deposited along with a host 147 selected from semiconductors ZnS,ZnMgS. The dots are n-type. In case the dots are SiOx cladded, effortsare made to remove or desorb the oxide cladding to deposit the host 147which electrically separates dots from each other. Layer 148 is a thinbuffer layer selected from lightly doped n-ZnS, ZnMgS. Another layer 150of Si dots 149 (having different size and band gap than dots in layer146) is deposited on the buffer layer 148. The dots are hosted in amatrix 151, which is selected from a list such as ZnS, ZnMgS, and othercompatible semiconductors. Layer 152 (shown as n-ZnS) forms the highlyconducting cap layer on which back Ohmic contact 153 is formed. Ifneeded an appropriate RTS interface may be introduced in this structurelike FIG. 9 b.

Furthermore, as shown in FIG. 11, a front illuminated cell havingquantum dot on Si substrates is further contemplated (Polycrystalline Sisubstrates are also envisioned).

FIG. 12a illustrates a methodology in which a dislocation-minimizedlateral epitaxial overgrowth (LEO) of a Ge layer on a Si substrate,using Group II-VI material buffer layers, is illustrated. It should beappreciated that a Ge layer may be used as a starting point for tandemcells realized in III-V and II-VI semiconducting layers as shown anddiscussed in FIGS. 4-6 and others. The method(s) disclosed hereinenables the fabrication of high efficiency solar cells, having multipletandem cells, using materials such as Si that are abundant in nature.Both polycrystalline and single crystal cells are also envisioned. Atypical process may incorporate the following steps:

-   -   (a) Pattern SiO₂ (161) is grown on a Si substrate (160) using 22        nm or smaller size lithography;    -   (b) Grow or deposit at low temperatures a buffer ZnS layer 162,        a ZnMgS layer 163, a graded ZnSSe layer 164 and finally a ZnSe        layer 165.    -   (c) Heat treat the grown II-VI layers (such as described for        post heteroepitaxial patterning, PHP) to glide dislocations from        the II-VI transition layers into SiO₂ walls;    -   (d) A Ge layer is grown on layer 166 between openings in SiO₂        layer 161. This is followed by lateral epitaxial overgrowth        (LEO) 166 of this layer on adjacent SiO₂ regions. Depending on        the LEO characteristics [8], the material for layer 166 can be        selected from Ge, ZnSe, ZnSSe, GaAs, and GaInAs. The primary aim        is to have dislocation density in layer 166 and 167 to be as        small as possible. This layer is now used as a high quality Ge        base layer on which tandem cells like FIG. 4-6 are grown, and    -   (e) Build lattice-matched III-V 168 and II-VI 169 solar cell        structures on layer 167. Here details of layers 168 and 169 are        not shown.

The layer 167 forms the base layer over which Groups III-V and II-VImaterial layers, configured as tandem solar cells, are grown.

Referring to FIG. 12b , SiO_(x)—Si quantum dots are self-assembled insmaller islands over p-doped regions [17] than is possible bylithography using site-specific self-assembly. In this case, the n-Sisubstrate 205 may first be patterned in n- and p-regions (206) using ionimplantation in the lithographed SiO₂ (not shown here). The dimensionsof p-regions may be laterally increased (and n-region reduced thanobtained by lithography) by using rapid thermal annealing (RTA) causinglateral diffusion of p-impurities. SiO_(x)-cladded Si nanodots (207) arethen self-assembled on the p-regions 206 forming a layer 210 consistingof many dot layers (two dot layers are shown; SiOx cladding 209 over aSi dot, like an isolated dot 208, is not shown explicitly). Following anannealing step, the p-region has SiO_(x)—Si layers whose thickness maybe dependent on the time of self-assembly. Now various Group II-VImaterial layers (162, 163, 164, and 165, as shown in FIG. 12a ) aregrown on the n-regions having no SiOx-Si dots. Selective area epitaxialregions consisting of 162-165 may now be processed (e.g. annealed),where this processing slides the dislocations existing in 162-165 layers(due to lattice mismatches and other causes) into the oxide regions.This may be followed by deposition of layer 166 (selected from GroupIII-V, II-VI, and Ge), and eventual lateral epitaxial overgrowth overthe regions 210. In this figure, we have shown a Ge layer grown on theZnSe cap. Like FIG. 12a , III-V and II-VI solar cells can be formed onlayer 167.

In accordance with the invention, details of an alternate processingsequence are outlined below:

Process Step I:

-   -   (a) Pattern n-Si substrate using 45 nm patterns (as an example)        on SiO₂;    -   (b) Ultra-low energy (ULE) p-type implant and rapid thermal        annealing (RTA) to form larger 67.5×67.5 nm (as an example) than        lithographed (45×45 nm) p-regions (and smaller 23 nm×23 nm than        lithographed n-regions); (in one embodiment squares of n-regions        surrounded by p-regions are formed);    -   (c) Remove SiO₂ mask and grow ZnS/ZnMgSe layers and other ZnSSe        transition layers.

Process Step II:

-   -   (d) Open larger islands 67.5 nm×67.5 nm (45+22.5 nm=67.5 nm)        using alignment marks over the p-Si doped regions;    -   (e) RIE etching of the II-VI layers to Si (p-Si doped layer);    -   (f) Deposit SiO₂ or self-assemble SiOx-cladded Si nanocomposites        of appropriate thickness; and    -   (g) Heat treat to consolidate SiOx-cladded Si nanodots to from        pin hole free SiOx layer. Continue heat treatment to glide        dislocations in II-VI regions now left on n-Si.

Process Step III:

-   -   (h) Nucleate Ge layers on ZnSe (or ZnSSe) layer using, for        example, ultra-high vacuum chemical vapor deposition (UHV-CVD)        or other technique(s) suitable for achieving lateral epitaxial        overgrowth on SiOx-Si regions surrounding epitaxial II-VI        layers. III-V layers are also envisioned on II-VI layers to form        epitaxial overgrowth. This could be followed by Ge growth        forming the base layer for subsequent III-V and II-VI solar        cells.

Referring to FIG. 13, a tandem cell with three p-n homojunction cellsusing Ge, GaAs, and GaInP and one heterojunction p-ZnSTe or p-ZnCdSe(layer 111)/n+-ZnSe or ZnSSe (211) cell [see FIGS. 5a, 6a, and 6b withvarious layers forming RTS (16, 17, 150) and tunnel junction cells (18,190)]. This cell can be further integrated with a ZnCdSe/ZnSSe quantumdot cell (#5). Layer 211 is deposited with p+-ZnSe (212) to form alow-voltage drop tunnel junction. Now one layer 215, hosting quantumdots 213 (selected from semiconductors selected from ZnSeTe, ZnCdSe) ina matrix (214) (semiconductor selected from ZnSe, ZnSSe, ZnMSSe), isformed. Similarly, another layer 217 hosting quantum dots 216 in amatrix 218, is formed on layer is deposited on a layer 219 whichseparates the two quantum dot consisting layers. Multiple layers ofquantum dots are envisioned depending on the method of deposition. Layer220 is the n+ZnSe cap layer on which top Ohmic contact grid 22 and ARcoating 23 is formed. The layers hosting quantum dots would absorbsunlight beyond the energy gap of pZnSTe or ZnCdSe (in cell 4). Cell 5is formed between p+ZnSe layer 212 and the n+ZnSe cap layer 220. Theselayers sandwich layers of quantum dots 215 and 217. It should beappreciated that the quantum dots can also be formed using selectivearea epitaxial growth in nanopatterned SiO₂ (as described in FIG. 12)using lateral epitaxial overgrowth techniques. One such technique hasbeen used to grow nanowires [17].

Referring to FIG. 14a , it should be appreciated that patternednanostructure grating may serve as antireflection coatings for broaderspectral ranges. These patterned 2-D gratings are shown in FIG. 14a . Aprior art 2-D grating is shown in FIG. 14b [15, 16]. It should befurther appreciated that use of self-assembled SiOx-cladded Si andGeOx-cladded Ge dots can be used for the formation of two-dimensionalgratings. Grating elements 221 consist of SiOx-Si or GeOx-Ge nanodots(222). One novelty in the use of cladded nanodots is their ability toadjust their effective dielectric constant. The effective dielectricconstant of a GeO_(x)—Ge nanodot layer can be adjusted in the range of8-14, depending on the cladding thickness and dot diameter. Variousgrating parameters such as grating periods (L1 and L2), grating ridgeheight d1, total grating thickness d2, will determine the antireflectiveproperties as a function of the spectral wavelength regime. FIG. 14cshows a layered structure where uniform dielectric layers, with varyingindex of refraction, are deposited. For example, the assembly ofGeOx-cladded Ge nanodots serves as a high dielectric constant layer 224.This layer is deposited on top of the outermost semiconductor layer 223in the solar cell structure. Note that the dielectric constant of layer224 is less than layer 223. Another layer 225 (comprising ofSiO_(x)-cladded Si nanodots) is deposited on top of layer 224. Note thatthe effective dielectric constant of GeO_(x)—Ge and SiO_(x)—Si dotsdepends on the core and cladding diameters. They can be changed between10-12, and 6-8, respectively. The nanodot layer 225 is deposited bystill lower dielectric constant SiON film 226 and subsequently by SiO₂film 227. A combination of uniform index nanodot layers as well aspyramidal dots (SiO_(x)—Si dots) can also be used to obtain a broaderspectral range grating.

In another embodiment, broad spectral range antireflection coating inconfigurations reported in the literature [25], cam be realized usingcladded nanodots along with other layers.

FIG. 15a shows layers (229, 230) of InGaAs grown on Si substrate (160)integrating dislocation reduction methodology similar to that shown inFIG. 12 (where the growth of Ge base layer 166 and 167 is illustrated).The InGaAs-on-Si solar cell structures incorporates InGaAs cell,AlInAs—InP cell, and ZnSeTe cells. This is in contrast to cellsillustrated in FIGS. 4-6. A typical process may incorporate thefollowing steps: Similar to FIG. 12a , a SiO₂ layer is deposited andpatterned (161) on a Si substrate (160) to obtain desired feature size(e.g., 22 nm or smaller to mitigate dislocations due to latticemismatch). This followed by growth of thin buffer ZnS layer 162, a ZnMgSlayer 163, and a graded ZnS to ZnSSe layer 164, and finally a ZnSe layer228. Heat treatment of the grown II-VI layers (such as described forpost heteroepitaxial patterning, PHP) may be used to glide dislocationsfrom the II-VI transition layers into SiO₂ walls. A ZnSe-to-ZnSeTegraded layer 229 is deposited with a thin ZnSe buffer 230. A heattreatment to glide dislocation into SiO₂ boundaries surrounding theII-VI layers is carried out. (In some cases, one heat treatment maysuffice following ZnSeTe graded layer 229). An InGaAs layer 231 is grownon ZnSe layer 230 (or 229 ZnSeTe layer depending on the epitaxialprocess) between openings in the SiO₂ layer 161. This is followed bylateral epitaxial overgrowth (LEO) 232. Depending on the LEOcharacteristics, the material for layer 232 can be selected from InGaAs,InP, AlInAs, or AlInAsP. InP layer 233 may be needed to planarize theLEO. The primary aim is to have dislocation density in layer 164 and229/230 to be as small as possible.

Layers 232/233 is now used as a high quality base layer on which tandemcells are grown. The first cell #1 being InGaAs cell with thin p+InGaAscontact layer 234, p-InGaAs absorber layer 235, and n-InGaAs layer 236forming the n-p junction. The n+InGaAs layer 237 and p+InGaAs layer 238forms the first tunnel junction. This is followed by p-InP absorberlayer 239 and an n-InP layer 240 enabling the formation of n-p InP cell(#2). Layers 241 (n+InP) and 242 (p+InP) form the second tunneljunction. A layer of p-ZnSTe 243 and an n-type ZnSeTe (or ZnMgSTe) layer244 is grown to obtain cell #3. The tandem 3-cell structure does notshow top and bottom contacts and AR coating. The introduction of RTSinterfaces is not shown explicitly. However, these RTS interfaces(comprising two barriers and a well) may be introduced between layers242 (p+InP) and 243 (pZnSeTe) and 238 (p+InGaAs) and 239 (p-InP) toreduce voltage offsets.

Alternately, the structure of FIG. 15a can be implemented usingsite-specific self-assembly methodology of FIG. 12 b.

FIG. 15b shows InGaAs FETs, realized on Si substrate 160, using ap-InGaAs layer 245 [in turn grown on InP layer 233 (shown in detail inFIG. 15a )]. The p-InGaAs layer 245 has n-type source (246) and drain(247). The gate region is shown with lattice-matched insulator 248, onwhich two layers of GeOx-cladded-Ge dots (249) are assembled. The gatelayer 250 and gate contact 251 are also shown. The source 252 and drain253 contacts are formed using AuGeNi. Other materials may be used. Theinset in FIG. 15b shows details of InGaAs FET which has quantum dotgate. These quantum dot gates can be used for memory and 3-state deviceoperation. The part of Si where there is no patterning and InGaAslayers, a conventional Si FET 254 (scaled down to 30 nm) is integrated.This is an example of achieving integration of Si electronics withInGaAs—InP FET based very high frequency (above 500 GHz) devices, aswell as solar cells. The solar cells are not shown in this figureexplicitly.

FIG. 16 illustrates tandem cells using p-Si substrate (like 160 in FIG.12a or with different specifications) forming the bottom Si homojunctioncell 255 (comprising of p-Si substrate 160, n-Si layer 256, and thehomojunction between them), an n+Si layer 257 and p+Si layer realizingthe tunnel junction 259, and a RTS structure 260. The RTS in turncomprises 261 ZnS barrier layer, a GaAsP well 262, and another barrierlayer 263 (which may be selected from GaP, AlGaP, AlGaAsP). The RTSfacilitates current transport between Si cell and the GaAsP cell. Layer263 is deposited with p-GaAsP thin layer (264). Layer 264 is depositedon with either a p-GaAsP layer (serving as the absorber of photons) orwith p-GaP/p(GaAsP—GaP) multiple quantum wells (265). Here, the noveltyis in the use of GaAsP—GaP multiple quantum wells (with barrier beingGaP layer 266 and well being GaAsP 267). Since GaAsP does not latticematch with Si or GaP, we use a compressive strained quantum well (40-60Angstroms) sandwiched between GaP barriers (50-80A). Many periods ofMQWs are shown. The window region is n-layer 268 (n-GaP forheterojunction) or AlGaP with appropriate band gap and strain. Thestrain may be matched by use of tensile strained barrier. The totalnumber of periods could be 50-70 which will create a region photons canbe absorbed at wavelengths in a range of about 1.8-2.0 eV (depending onthe composition). The Si homojunction cell will absorb radiation in the1.1-1.8 eV spectral regime. The n+GaP can be used as a cap layer (269)to form top Ohmic contacts (not shown) or to form a n+/p+GaP tunneljunction (not shown). This can be followed by the formation of a II-VIcell (using ZnSSe and ZnMgS layers).

FIG. 17a shows homojunction and heterojunction cells integrated with RTSand tunnel junction interfaces. Here, a Ge substrate 141 with p+Ge layer1410 (to facilitate formation of back Ohmic contact 21) is used. ThenGaAs layer 29 (FIG. 4c ) forms the heterojunction 28 with pGe (141) andthe combination works as a cell #1. Subsequently, layers 191 and 192 areincorporated to form a tunnel junction 19. Layer 132 (p-GaAs) serves asthe absorber for the homojunction cell #2 (whose window region is n-GaAslayer 13; see FIG. 6a ). Layer 132 of n-GaAs is contacted with a RTSinterface 270, which is realized by barrier 271 (n-ZnSe, n-GaInP) andwell layer 272 (InGaAs), and another barrier layer 273 (n-GaInP, bandgap ˜2 eV). This is followed by layer 1910 n+GaInP and layer 1920p+GaInP together forming the tunnel junction # (190). Layer 1920 haslayer 121 of p-GaInP which serves as the absorber of ˜1.86 eV radiation.Together with layer 122, layer 121 forms a homojunction 27 and cell #3.Layer 274 provides a cap layer to form top Ohmic contact 275.Antireflection coating 276 is shown as well.

FIG. 17b shows a structure where a RTS (277) is incorporated at theheterojunction (nGaAs-pGe) to facilitate charge transport. The RTS 277comprises a barrier 54 (ZnSe), well 55 (InGaAs) and another barrierlayer 56 (GaInP). Combination of RTS, heterojunctions, and homojunctionsare envisioned to reduce the effect of band-offsets. In thesestructures, we can incorporate II-VI (ZnSe-based) cells to convertphotons that are not effectively absorbed in GaInP (similar to shown inFIGS. 4-6).

FIG. 18a shows a Si cell [comprising of layers 281 (p+Si thin layer),280 (p-Si photon absorbing layer) and 282 (n+-Si window region)], whichis integrated with dislocation reduction structure providing interfacefor III-V and II-VI tandem cells. The dislocation reducing structure issimilar to that of FIGS. 12a and 12b . Here, we use a nanopatterned SiO₂layer 161. The Si surface of layer 283, where there is no oxide, isdeposited with n+ZnS or ZnMgS buffer layer 283 (unlike the layer 162 ofFIG. 12a , here the layer 283 is shown doped). This is followed by agraded n+ZnSSe layer 284. Again this layer is n+ doped (see itscounterpart 164 which is not shown doped). Upon this layer we haven+ZnSe (285). This deposition is followed by the heat treatmentdiscussed before (in context to FIG. 12) to remove dislocations causedby lattice mismatch. This is followed by deposition of thin n+GaAs layer286 which nearly fills the SiO₂ surrounded regions and grows over SiO₂regions 161. Finally, layer 287 (p+GaAs) is grown over the lateralepitaxial overgrowth over the entire region forming a p+/n+ tunneljunction. This is followed by the growth of p-GaAs (or p-GaInP orp-AlGaAs with appropriate band gap) as the photon absorber layer 288.Layer 289 (selected from n-GaAs, n-GaInP or n-AlGaAs) is deposited toform either a hetero or homojunctions. The band gap of the absorberlayer determines the spectral range of photons that are absorbed by theSi cell underneath. Layer 289 has on top of it a thin n+ AlGaAs orn+GaInP (layer 290) and a thin p+AlGaAs or p+GaInP (291) layer. The twolayers form the tunnel junction. Additionally (like FIG. 6a ), a II-VIcell of ZnSTe (layers 111, 1120) can be fabricated. Here, RTS structure150 may be disposed between layer 291 and layer 111. This figure alsoshows p-Si layer 292 which is doped n+ (293) in regions where SiO₂ layer161 has openings. Layer of ZnS (283) is grown on n+Si regions 293. FIG.18b shows a Si cell integrated with dislocation reduction methodologyintegrating structure consisting of SiOx-Si cladded dots providinginterface for III-V and II-VI tandem cells. SiOx-cladded Si dots (294)are self-assembled on the surface of p-Si regions (295) in the layer 292(where n+ doping is not there).

It should be appreciated that simulation of tandem solar Cells may beneeded to successfully fabricate Ge, Group III-V, and Group II-VIstructures. Additionally, tandem laser structures which incorporatetunnel junctions have been reported [18, 19] and they can form guides tothe design of high concentration solar cell designs. It is contemplatedthat cell modeling can be accomplished using known design equations.

Additionally, fabrication of Group II-VI heterojunction cells may beaccomplished, where Group II-VI layers have been in various combinationson GaAs, InGaAs—InP, Ge and Si substrates [21,22]. ZnSe—GaAs cells[23,24] and n-ZnMgS-pSi and ZnSSe—Si solar cells may be fabricated [24].It should be appreciated that lattice matched wide-energy gap GroupII-VI layers (such as ZnMgS-on-Si or ZnSSe-on-GaInP) serves at least twofunctions: (1) wide energy gap window region to ensure low-leakagecurrent and (2) antireflection coating when its thickness isappropriately chosen particularly for the peak wavelength spectralregime. The zinc sulfoselenide and ZnMgS material systems offer a widerange of direct bandgaps for the fabrication of Si-based tandem solarcells.

Furthermore, growth of ZnSe—ZnSSe cladding on Ge dots may beaccomplished, where GeO_(x) cladding from Ge dots may be removed andreplaced by n-ZnSe layer that may be grown using metalorganic chemicalvapor phase deposition (MOCVD) (or some other suitable method). Thisprovides needed cladding to separate Ge dots to realize a highabsorption coefficient α of ˜100,000 cm⁻¹. A second set of largerdiameter (lower effective band gap Eg2) Ge dots may be self-assembledwith a ZnSe cladding layer. Also, fabrication of Quantum Dot Solar Cellsmay be accomplished where a single junction solar cell can be fabricatedby depositing a thin GaAs or GaInP layer over ZnSe layer hosting Gequantum dots (Eg3). Processing steps for Ohmic contacts may be anyprocessing step suitable to the desired end purpose, such as similar tothose known for ZnSe—GaAs cells.

Referring to FIG. 20, the first solar cell 2001 is constructed with atleast two layers. The first layer is p-Type Si 2003 and a second n-typelayer 2004. In one embodiment the first p-type layer 2003 is used as asubstrate which is selected one from single crystalline, polycrystalline and nano-crystalline Si. The p-type first layer 2003 offirst solar cell 2001 is disposed with an Ohmic contact 2016. The solarcell structure 2000 comprises a tunnel junction 2005 and resonanttunneling structure 2008 between the first solar cell 2001 and thesecond solar cell 2002. The tunnel junction 2008 is comprised of an n+type layer 2006 and a p+-type layer 2007. The semiconductor layers 2006and 2007 forming the tunnel junction 2005 are selected one from Ge, Si,CdSeTe, CdGalnSe. In one embodiment first tunnel junction is comprisedof n+Si layer 2006 on top of second n-type layer 2004 of first solarcell 2001. The p+ type layer 2007 of the tunnel junction is depositedwith a resonant tunneling structure 2008. The resonant tunnelingstructure comprises a first quantum barrier layer 2009, a second quantumwell layer 2010, and a third quantum barrier layer 2011. The firstquantum barrier layer 2009 is constructed at least one of ZnTe, ZnCdTe,ZnSTe, ZnSeTe. The second quantum well layer 2010 is constructed atleast one of ZnCdTe, CdTe, and the third quantum barrier layer 2011 isconstructed at least one of ZnTe, ZnCdTe, and ZnSTe.

In one embodiment, there is a buffer layer 2012 between the thirdquantum barrier layer 2011 of resonant tunneling structure 2008 and thesecond solar cell 2002. The second solar cell 2002 comprises a thirdp-layer 2013 and a fourth n-type layer 2014. The third p-type layer 2013is selected one from CdTe, ZnCdTe, CdInGaSe (CIGS), and wherein thefourth n-type layer is selected from CdS, ZnCdS, CdGalnSe. Disposed onfourth n-type layer 2014 of second solar cell 2002 is a top Ohmiccontact grid 2015. In one embodiment, the fourth n-type layer 2014 ofsecond solar cell 2002 is deposited with an Ohmic contact facilitatorlayer 2017. The n-type layer 2017 is one selected from low conductivityCdS, ZnCdS, CdGalnSe.

Referring to FIG. 21 a solar cell structure 2020, comprising of twosolar cells, a first solar cell 2021 and a second solar cell 2022. Thefirst solar cell 2021 is constructed with at least two layers. The firstsemiconductor layer is n-type CdS 2023 and a second p-type layer 2024 isCdTe. In one embodiment the first n-type layer 2023 is deposited on atransparent conducting oxide (TCO) 2025 which in turn deposited on asubstrate 2026. The substrate 2026 is one selected from glass, quartz,and sapphire. The TCO layer 2025 forms the top Ohmic contact for thesolar cell structure 2020. The solar cell structure 2020 comprises aresonant tunneling structure 2027 and a tunnel junction (TJ) 2028disposed between the first solar cell 2021 and the second solar cell2022. The resonant tunneling structure 2027 comprises a first quantumbarrier layer 2029, a second quantum well layer 2030, and a thirdquantum barrier layer 2031. The first quantum barrier layer 2029 isconstructed at least one of ZnTe, ZnCdTe, ZnSTe, ZnSeTe. The secondquantum well layer 2030 is constructed at least one of Ge, ZnCdTe, CdTe,and the third quantum barrier layer 2031 is constructed at least one ofZnTe, ZnCdTe, and ZnSTe. The tunnel junction 2028 is comprised of a p+type layer 2032 and an n+-type layer 2033.

The p-type layers 2032 is one selected one from CdTe, ZnTe, Ge, Si,ZnCdTe, and CdGalnSe. The n-type layer 2033 is one selected from CdS,Ge, Si, ZnCdS, and CdGaInS. In one embodiment, there is a buffer layerbetween the third quantum barrier layer 2031 of the resonant tunnelingstructure 2027 and layer 2032 of the tunnel junction 2028. The secondsolar cell 2022 comprises a third n-type layer 2034 and a fourth p-typelayer 2035. The third n-type layer 2034 is selected one from Ge and Si.Disposed on fourth p-type layer 2035 of second solar cell 2022 is anOhmic contact facilitator layer 2036. The p+-type layer 2036 is oneselected from low conductivity Ge and Si. The facilitator layer 2036 isdisposed with bottom Ohmic contact 2037.

Referring to FIG. 22 the solar cell structure 20201 is similar to thatof FIG. 2A in terms of first solar cell 2021. The difference is in thesecond cell. The second solar cell 2040 comprises a first n-type layer2038 and a second p-type layer 2039. The first n-type layer 2038 is oneselected from n-type amorphous Si, n-type amorphous Ge, n-typepolycrystalline Si, and n-type polycrystalline Ge. The second p-typelayer 2039 comprises more than one p-type interface layers and more thanone quantum dot sub-layers. Shown in the figure are quantum dotsub-layer 20391 and quantum dot sub-layer 20392. The first interfacelayer 20390 and second interface layer 2039 are one selected from p-typeamorphous Si, p-type amorphous Ge, p-type polycrystalline Si, and p-typepolycrystalline Ge. The quantum dot sub-layers 20391 and 20392 arecomprised of cladded quantum dot layers. Each of these sub-layers havingassembly of quantum dot comprises more than one cladded quantum dot persub-layer. Two individual quantum dot layers are shown in the figure foreach quantum dot layer 20391 and 20392. The cladded quantum dot layersare comprised of GeOx cladding and Ge nanodot core. The core of thequantum dot is 2-6 nm in average diameter and the cladding thickness isin the range of 0.5-1.5 nm. The assembly of quantum dots form a quantumdot superlattice (QDSL) layer with its characteristics mini-energybands.

Sandwiched between quantum dot sub-layer 20391 and sub-layer 20392, inone embodiment, is a p-type interface 2040, which is selected one fromp-type amorphous Si, p-type amorphous Ge, p-type polycrystalline Si, andp-type polycrystalline Ge. Disposed on the quantum dot sub-layer 20392is a facilitator layer 2041 which facilitates in forming bottom Ohmiccontact 2042. The facilitator layer 2041 is one selected from p-typeamorphous Si, p-type amorphous Ge, p-type polycrystalline Si, and p-typepolycrystalline Ge.

Referring to FIG. 23 a solar cell structure disposed on a p-GaNsubstrate 3000, where a buffer layer 3001 is disposed to epitaxiallydeposit a p-GaN layer 3002, and disposed on p-GaN layer is one or moreInGaN quantum wells forming a set 3003, and InGaN quantum wells areseparated by a set of quantum barrier layers 3004 selected from GaN andAlGaN. Disposed on outer quantum barrier layer of set of quantum barrierlayer is an n-type GaN layer 3005. Here, p-type GaN 3002, set 3003 ofInGaN quantum well layer and set 3004 of barrier layers, and n-type GaNlayer 3005 forming the first solar cell 2999 on GaN substrate 3000.Disposed on n-type GaN layer of first solar cell is a nano-patternedregion layer 3006, comprising masked regions 3007 constructed from oneselected from SiO₂, Si₃N₄, SiO_(x)-cladded Si nanodot layer, and exposedregions 3008. The exposed regions 3008 exposes the n-type GaN layer 3005of first solar cell 2999 in the nano-patterned region layer 3006. Themasked regions 3007 in the nano-patterned region layer 3006 hasdimension in the range of ˜20 nm×20 nm to ˜100 nm×100 nm on n-type GaNlayer 3005 of first solar cell 2999. The thickness of oxide maskedregions 3007 in the nano-patterned layer is ˜10-100 nm.

The exposed regions 3008 on n-type GaN layer 3005 are deposited withfirst buffer layer 3009, selected one of n-Ge, n-Si, n-SiGe, n-ZnS,n-ZnMgS layer about 2-10 nm in thickness. The first buffer layer 3009 isdeposited with a second buffer layer 3010 and a third buffer layer 3011.The second buffer layer and third buffer layers are semiconductor layersselected from n-type doped layers of Ge, Si, SiGe, CdS, ZnSSe, ZnSe,ZnMgTe, GaInP, CdTe, ZnCdSTe. In one embodiment the second buffer layer3010 maybe a graded layer between first buffer layer 3009 and thirdbuffer layer 3011. In one embodiment second buffer layer has acomposition varying from Si to Ge, ZnS to ZnSSe, ZnBeS—ZnSSe, ZnSeTe toZnCdTe, or combination of these. The third buffer layer 3011 isnucleated on the second buffer layer 3010 and spreads over (like lateralepitaxial overgrowth) on the mask regions 3007 of the nano-patternedregion layer 3006. In one embodiment, the first buffer layer, secondbuffer layer and third buffer layer serve as first resonant tunnelingstructure 3013 where first buffer layer serves as first quantum barrierlayer and second buffer layer serves as second quantum well layer andthe third buffer layer as third quantum barrier layer.

The third buffer layer 3011 has fourth buffer layer 3012 whichinterfaces via the second buffer layer 3010 in transporting electronsfrom n-GaN layer 3005. A first resonant tunneling structure 3013 andfirst tunnel junction 3014 are located between a first solar cell 2999and a second cell 3017, and second solar cell is constructed from p-typelayer 3018 and n-type layer 3019. In one embodiment both 3018 and 3019layers are constructed from AlGaInP. In one embodiment the first tunneljunction 3014 is constructed from n+ type layer 3015 and p+ type layer3016. The n+ type layer 3015 and p+ type layer 3016 are selected onefrom AlGaInP, GaInP, and GaAs. The p-type layer 3018 of second solarcell 3017 is disposed on p+ type layer 3016 of first tunnel junction3014.

A second resonant tunneling structure 3020 and a second tunnel junction3024 and are located between the second solar cell 3017 and the thirdcell 3027. The second resonant tunneling structure 3020 includes afourth quantum barrier layer 3021, a fifth quantum well layer 3022, anda sixth quantum barrier layer 3023. The fourth quantum barrier layer3021 is one selected from AlGaInP, AlGaAs, and ZnSSe, and the fifthquantum well layer 3022 is one selected from GaAs, GaInP, and AlGaAs,and the sixth quantum barrier layer 3023 is one selected from one ofAlGaInP, ZnSe, and ZnSTe. The second tunnel junction 3024 is comprisedof an n+ type layer 3025 and a p+ type layer 3026, and 3025 and 3026 areselected one from GaInP and GaAs. The third solar cell 3027 isconstructed from p-type layer 3028 and n-type 3029 GaInP layers forminga p-n junction. The p+ layer 3026 of second tunnel junction 3024 isdisposed with sixth quantum barrier layer 3023 of second resonanttunneling structure 3020.

A third resonant tunneling structure 3030 and a third tunnel junction3034 are located between the third solar cell 3027 and the fourth solarcell 3037. The third tunneling structure 3030 comprises the seventhquantum barrier layer 3031, the eighth quantum well layer 3032 and aninth quantum barrier layer 3033. The seventh quantum barrier layer 3031is constructed from at least one of GaInP, AlGaAs, AlGaInP and theeighth quantum well layer 3032 is constructed from at least one of GaAs,GaInAs, and the ninth quantum barrier layer 3033 is constructed from oneselected from GaInP, AlGaAs and AlGaInP. The third tunnel junction 3034comprises an n+ type layer 3035 and a p+ type layer 3036. The fourthcell 3037 comprises of a p-type layer 3038 and an n-type layer 3039. Thep− type layer 3038 and n-type layer 3039 are constructed from oneselected from GaAs and GaInAs. The n-type layer 3039 is disposed with ann+ type layer 3040 constructed from GaInAs to facilitate bottom or backOhmic contact 3041. Furthermore, the first solar cell 2099 has an Ohmiccontact 2098 formed on the p-GaN layer 3002 forming the front or topOhmic contact 2998 in the form of a grid permitting light through thesapphire substrate. The GaN substrate 2999 is textured or deposited witha dielectric coatings which serves as an antireflection coating.

Referring to FIG. 24 a solar cell structure similar to FIG. 23 with theprimary difference being in the substrate used is sapphire 2997. Inaddition, the top Ohmic contact grid 2996 is formed on p-GaN layer 3002disposed on GaN buffer layer 3001 which in turn is deposited on thesapphire substrate 2997. The bottom Ohmic contact is disposed on then+-type facilitator layer 3040 which is deposited on the second n-typelayer 3039 of the fourth solar cell 3037. Referring to FIG. 25 a solarcell structure similar to that of FIG. 23 and FIG. 24 with the primarydifference being in the construction of first p-type layer and secondn-type layer of one or more of plurality of solar cells. In FIG. 23 andFIG. 24 the second solar cell 3017 is shown to comprise of two layers;first p-type layer 3018 and second p-type layer 3019. Both of theselayers are shown to be made of AlGaInP. However, in the solar cellstructure of FIG. 25 the second solar cell 3017 is shown with firstp-type layer comprising of three sub-layers 3018-1 (first sub-layer),3018-2 (second sublayer), and 3018-3 (third sub-layer). Similarly, thesecond n-type layer comprises of three sub-layers 3019-1 (firstsub-layer), 3019-2 (second sub-layer), and 3019-3 (third sub-layer). Inone embodiment, p-type sublayer 3018-1 is constructed from AlGaInP suchthat it has an energy gap of E_(g1), and p-type sub-layer 3018-2 isconstructed from AlGaInP with a composition resulting in an energy gapof E_(g2), and the p-type sub-layer 3018-3 is constructed from AlGaInPwith an energy gap of E_(g1). Here, p-type sub-layer 3018-2 has E_(g2)which is higher in magnitude than E_(g1) (i.e. E_(g2)>E_(g1)).Similarly, the three n-type sub-layers 3019-1, 3019-2, and 3019-3 areconstructed from AlGaInP and have their energy gaps E_(g1), E_(g2), andE_(g1), respectively.

Although not shown in FIG. 25, the third solar cell 3027 (constructedfrom GaInP) could be configured by replacing first p-type layer 3028 andsecond n-type layer 3029 by three sub-layers 3028-1/3028-2/3028-3 and3029-1/3029-2/3029-3, respectively. The second p-type sub-layer 3028-2and second n-type sub-layer 3029-2 are constructed from a higher energygap (E_(g4)) GaInP than the energy gap E_(g3) of first and thirdsub-layers. Referring to FIG. 26 a solar cell structure comprising aplurality of solar cells including first solar cell, a second solarcell, a third solar cell, a fourth solar cell and a fifth solar cell.The first solar cell comprises a first p-type layer 4000 and a secondn-type layer 4001. In one embodiment, the first p-type layer serves asthe silicon substrate 4000. The second n-type layer 4001 is disposedwith another n+-type layer 4002 to form an n+-n-p homojunction. Thus,first solar cell 3999 comprises of a substrate, second n-type layer anda third n+-type layer 4002. The second cell is comprised of either GaAsand/or GaInAs, the third cell is made of GaInP, the fourth cell is madeof AlGaInP, and the fifth solar cell is constructed from ZnSe andrelated semiconductors. The solar cell structure comprisesnano-patterned regions interfacing first Si-based solar cell with GaAs,GaInP, AlGaInP, and ZnSe base solar cells, generally lattice-matched toGaAs.

Disposed on third n+-type Si layer 4002 of first solar cell 3999 is anano-patterned region layer 4006, comprising masked regions 4007constructed from one selected from SiO₂, Si₃N₄, SiO_(x)-cladded Sinanodot layer, and exposed regions 4008. The exposed regions 4008exposes the n+-type Si layer 4002 of first solar cell 3999 in thenano-patterned region layer 4006. The masked regions 4007 in thenano-patterned region layer 4006 has dimension in the range of ˜20 nm×20nm to ˜100 nm×100 nm on n+-type Si layer 4002 of first solar cell 3999.The thickness of oxide masked regions 4007 in the nano-patterned layeris ˜10-100 nm. The exposed regions 4008 on n+-type Si layer 4002 aredeposited with first buffer layer 4009, selected one of n-ZnS, n-ZnMgS,n-ZnMgSSe layer about 2-10 nm in thickness. The first buffer layer 4009is deposited with a second buffer layer 4010 and a third buffer layer4011. The second buffer layer and third buffer layers are semiconductorlayers selected from n-type doped layers of ZnSSe, ZnSe, ZnMgTe, GaAs,and GaInAs. In one embodiment the second buffer layer 4010 maybe agraded layer between first buffer layer 4009 and third buffer layer4011. In another embodiment second buffer layer has a compositionvarying from ZnS to ZnSSe, ZnBeS—ZnSSe, ZnMgSSe—ZnSe, ZnSeTe to ZnCdTe,or combination of these. The third buffer layer 4011 is nucleated on thesecond buffer layer 4010 and spreads over (like lateral epitaxialovergrowth) on the mask regions 4007 of the nano-patterned region layer4006.

In still another embodiment, the first buffer layer 4009, second bufferlayer 4010 and third buffer layer 4011 serve as first resonant tunnelingstructure 4013 where first buffer layer 4009 serves as first quantumbarrier layer and second buffer layer 4010 serves as second quantum welllayer and the third buffer layer 4011 as third quantum barrier layer.The third buffer layer 4011 has fourth buffer layer 4012 whichinterfaces via the second buffer layer 4010 in transporting electronsfrom n+ Si layer 4002. A first resonant tunneling structure 4013 andfirst tunnel junction 4014 are located between a first solar cell 3999and a second cell 4017, wherein the second solar cell is constructedfrom p-type layer 4018 and n-type layer 4019. In one embodiment both4018 and 4019 layers are selected one from GaInAs and GaAs. In oneembodiment the first tunnel junction 4014 is constructed from n+ typelayer 4015 and p+ type layer 4016. The n+ type layer 4015 and p+ typelayer 4016 are selected one from GaInP, and GaAs. The p-type layer 3018of second solar cell 3017 is disposed on p+ type layer 3016 of firsttunnel junction 4014. A second resonant tunneling structure 4020 and asecond tunnel junction 4024 and are located between the second solarcell 4017 and the third cell 4027.

The second resonant tunneling structure 4020 includes a fourth quantumbarrier layer 4021, a fifth quantum well layer 4022, and a sixth quantumbarrier layer 4023. The fourth quantum barrier layer 4021 is oneselected from AlGaInP, AlGaAs, and ZnSSe, and the fifth quantum welllayer 4022 is one selected from AlGaInP, GaInP, and AlGaAs, and thesixth quantum barrier layer 4023 is one selected from one of AlGaInP,ZnSe, and ZnSTe. The second tunnel junction 4024 is comprised of an n+type layer 4025 and a p+ type layer 4026, and 4025 and 4026 are selectedone from GaInP and GaAs. The third solar cell 4027 is constructed fromp-type layer 4028 and n-type 4029 GaInP layers forming a p-n junction.The p+ layer 4026 of second tunnel junction 4024 is disposed with sixthquantum barrier layer 4023 of second resonant tunneling structure 4020.

A third resonant tunneling structure 4030 and a third tunnel junction4034 are located between the third solar cell 4027 and the fourth solarcell 4037. The third tunneling structure 4030 comprises the seventhquantum barrier layer 4031, the eighth quantum well layer 4032 and aninth quantum barrier layer 4033. The seventh quantum barrier layer 4031is constructed from at least one of GaInP, AlGaAs, AlGaInP and theeighth quantum well layer 4032 is constructed from at least one ofAlGaInP and GaInP, and the ninth quantum barrier layer 4033 isconstructed from one selected from AlGaAs and AlGaInP. The third tunneljunction 4034 comprises an n+ type layer 4035 and a p+ type layer 4036.The fourth cell 4037 comprises of a p-type layer 4038 and an n-typelayer 4039. The p-type layer 4038 and n-type layer 409 are constructedfrom one selected from AlGaInP. The fourth resonant tunneling structure(RTS) 4040 is comprised of tenth quantum barrier layer 4041, eleventhquantum well layer 4042 and twelfth quantum barrier layer 4043. Whereinthe tenth quantum barrier layer 4041 is one selected from ZnSSe,ZnMgSSe, AlGaInP, and eleventh quantum well layer 4042 is one selectedof ZnSe and AlGaInP, and twelfth quantum barrier layer 4043 isconstructed from ZnSSe, ZnMgSSe.

The fourth resonant tunneling structure 4040 is disposed with a fourthtunneling junction 4044. The fourth tunneling junction 4044 is comprisedof a p+-type layer 4045 and an n+-type layer 4046. The tunnel junctionlayers 4045 and 4046 are one selected from ZnSe and AlGaInP. The n+-typelayer 4046 is disposed with fifth solar cell 4047. The fifth solar cell4047 is comprised of first p-type layer 4048 and a second n-type layer4049. The n-type layer 4049 is disposed with an n+ type facilitatorlayer 4050, and the facilitator layer is constructed one from ZnSe andAlGaInP to facilitate top Ohmic contact 4051. The top Ohimc contact isconfigured in the form of a grid permitting light to be incident on thesolar structure. Furthermore, the first solar cell 3099 has an Ohmiccontact 3098 formed on the p-Si substrate 4000.

Referring to FIG. 27 a solar cell structure having a plurality of solarcells, operating in IR, red, green and blue spectral regime. The solarcell structure is disposed on p-Ge substrate 5000. In one embodiment, Geserves as the substrate for four other solar cells. In anotherembodiment, the p-type Ge substrate forms the first solar cell on whichresonant tunneling structures, tunnel junctions and four other solarcells are disposed. We describe this five-cell structure. The firstsolar cell 5003 is constructed of p-type Ge substrate 5000 and an n-typeGe layer 5001 and an n+-type layer 5002. The second solar cell 5011 isconstructed from GaAs and the third solar cell 5021 is constructed ofGaInP, and the fourth solar cell 5045 is constructed from AlGaInP, andthe fifth solar cell 5061 is constructed from ZnSe based semiconductorthin films. Disposed in between first solar cell 5003 and second solarcell 5011 is the first resonant tunneling structure 5004. The firsttunneling resonant structure comprises first quantum barrier layer 5005,second quantum well layer 5006, and third quantum barrier layer 5007,and wherein first quantum barrier layer is disposed on n+-type Ge layer5002. First quantum barrier layer is one selected from GaInP, AlGaInP,AlGaAs, and second quantum well layer is one selected of GaAs, GaInAs,and third quantum barrier layer is constructed from GaInP, AlGaInP,AlGaAs.

The first tunneling junction 5008 comprises of n+ GaAs 5009 and p+ typelayer 5010 of GaAs. The second solar cell 5011 comprise of two solarsub-cells, a bottom sub-cell 5012 and a top sub-cell 5013, and whereinthe bottom sub-cell 5012 is comprised of first p-GaAs layer 5014 and athin first n-GaAs layer 5015. A second tunnel junction 5016 separatesthe bottom sub-cell and top sub-cell. The second tunnel junction 5016 iscomprised of an n+-type layer 5017 and a p+-type layer 5018. The topsolar sub-cell 5013 of second solar cell has a second p-GaAs layer 5019and a second n-GaAs layer 5020. The second p-type GaAs layer 5019 isthinner than the first p-GaAs layer 5014 of bottom sub-cell 5012.Disposed in between second solar cell 5011 and third solar cell 5021 isthe second resonant tunneling structure 5022. The second tunnelingresonant structure 5022 comprises fourth quantum barrier layer 5023,fifth quantum well layer 5024, and sixth quantum barrier layer 5025. Thefourth quantum barrier layer 5023 is disposed on n-type GaAs layer 5020,and the fourth quantum barrier layer is one selected from AlGaInP,AlGaAs, and the fifth quantum well layer 5024 is one selected from GaInPand GaAs, and sixth quantum barrier layer 5025 is constructed fromAlGaInP and AlGaAs. The third tunnel 5026 having n+ type layer 5027 anda p+ type layer 5028. These layers are one selected from GaAs and GaInP.The third tunnel junction 5026 is disposed between the sixth barrierlayer 5025 of second resonant tunneling structure 5022 and the thirdsolar cell 5021.

The third solar cell 5021 constructed from GaInP and has a bottomsub-cell 5029 and a top sub-cell 5030. The bottom sub-cell 5029 has afirst p-type GaInP layer 5031 and first n-GaInP layer 5032 forming thebottom sub-cell 5029. A fourth tunnel junction 5033 separates the bottomsub-cell 5029 and top sub-cell 5030. The fourth tunnel junction 5033 iscomprised of an n+-type layer 5034 and a p+-type layer 5035. The stopsub-cell 5030 of the third solar cell 5021 has a second p-type layer5036 and a second n-type layer 5037 of GaInP. The second p-type GaInPlayer 5036 is thinner than the first p-type GaInP layer 5031. Disposedon the second n-type layer 5037 of the third solar cell 5021 is thethird resonant tunnel structure 5038. The third resonant tunnelingstructure comprises of seventh quantum barrier layer 5039, an eighthquantum well layer 5040, and a ninth quantum barrier layer 5041. Theseventh quantum barrier layer 5039 is one selected from AlGaInP andZnSe, and the eighth quantum well layer 5040 is one selected fromAlGaInP and GaInP, and the ninth quantum barrier layer 5041 isconstructed from AlGaInP and ZnSe.

Disposed on the ninth quantum barrier layer 5041 of the fifth tunneljunction 5042 which comprises a n+-type layer 5043 and on top of which ap+-type layer 5044. These layers are one selected from AlGaInP andGaInP. The n+-type layer 5043 of is disposed on ninth quantum barrierlayer 5041. The fourth solar cell 5045 is disposed on p+-type layer5044. The fourth solar cell 5045 is constructed from AlGaInP. The fourthsolar cell 5045 is constructed from AlGaInP and has a bottom sub-cell5046 and a top sub-cell 5047. The bottom sub-cell 5046 has a firstp-type AlGaInP layer 5047 and first n-AlGaInP layer 5048 forming thebottom sub-cell. A second p-type AlGaInP layer 5049 and a second n-typeAlGaInP layer 5050 forming the top sub-cell 5047 of the fourth solarcell 5045. The first p-type AlGaInP layer 5047 is thinner than thesecond p-type AlGaInP layer 5049.

A fifth tunnel junction 5051 separates the bottom sub-cell 5046 and topsub-cell 5047. The fifth tunnel junction 5051 is comprised of an n+-typelayer 5052 and a p+-type layer 5053 constructed from AlGaInP. Disposedon the second n-type layer 5050 of fourth solar cell 5045 is the fourthresonant tunneling structure 5054. The fourth resonant tunnel structurecomprise of tenth quantum barrier layer 5055, eleventh quantum welllayer 5056, and twelfth quantum barrier layer 5057. Wherein the tenthquantum barrier layer 5055 is one selected from ZnSSe, ZnMgSSe, AlGaInP,and eleventh quantum well layer 5056 is one selected of ZnSe andAlGaInP, and twelfth quantum barrier layer 5057 is constructed fromZnSSe, ZnMgSSe. The twelfth quantum barrier layer 5057 is disposed withsixth tunnel junction 5058, comprising of a n+ layer 5059 and a p+ typelayer 5060. The tunnel junction 5058 layers are one selected from ZnSeand AlGaInP. The fifth solar cell 5061 comprises a bottom sub-cell 5062and a top sub-cell 5063. The bottom sub-cell 5062 has a first p-ZnSelayer 5063 and a first n-ZnSe layer 5064, and the top sub-cell comprisesof a second p-ZnSe layer 5065 and a second n-ZnSe layer 5066. The firstp-ZnSe layer 5063 of bottom sub-cell is thicker than second p-ZnSe layer5065 of top sub-cell 5063. The fifth solar cell 5061 is disposed on thep+-type layer 5060 of sixth tunnel junction.

A seventh tunnel junction 5067 separates the bottom sub-cell 5062 andtop sub-cell 5063. The seventh tunnel junction 5067 is comprised of ann+-type layer 5068 and a p+-type layer 5069. The seventh tunnel junction5067 is constructed from one selected from ZnSe and AlGaInP. The secondn-type layer 5066 of fifth solar cell 5061 has an n+-type layer 5070facilitating the formation of top Ohmic contact grid 5071 on secondn-type layer 5066. The bottom Ohmic contact 4999 is formed on p-Gesubstrate 5000. Referring to FIG. 28 a GaAs solar cell comprising of abottom solar sub-cell, tunnel junction and a top solar sub-cellfabricated on an un-doped GaAs substrate. A solar cell structure 6015comprising of a bottom sub-cell 6005 and a top sub-cell 6009. The solarstructure is comprised of un-doped GaAs substrate 6000. The substratehas a buffer layer 6001. Disposed on buffer layer is an n+-type GaAslayer 6002. The n+-type layer is disposed with an n-type layer 6003 anda p-type layer 6004. Layers 6002, 6003 and 6004 forms the bottomsub-cell 6005. Disposed on p-type layer 6004 is a tunnel junction 6008.The tunnel junction is comprised of a p+-type layer 6006 and an n+-typelayer 6007. The tunnel junction layers are thinner than the sub-celllayers 6004 and 6003. Disposed on the n+-type layer 6007 of tunneljunction 6008 is the n-type layer 6010 of top sub-cell 6009. The topsub-cell 6009 is comprised of n-type layer 6010 and p-type layer 6011.The p-type layer is disposed with an Ohmic contact grid 6012. The bottomOhmic contact 6013 is formed on layer 6002. A mesa 6014 is formed byetching various layers to expose n+-type GaAs layer 6002.

Referring to FIG. 29 the voltage-current (V-I) plot of a GaAs solar cellwith two sub-cells. The current I in mA is the vertical axis and voltageV in Volts is the horizontal axis. The illumination was done using awhite light lamp. Referring to FIG. 30 a schematic block diagram showingfour solar cell structure interfaced to a switching block controlled bya microprocessor. The bidirectional converter enables powering of a load(or battery) using sunlight or connecting solar cell structure in a modewhen it serves as a light emitting device.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention.Moreover, the embodiments or parts of the embodiments may be combined inwhole or in part without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the invention isnot limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. It should be appreciated that although the invention isdescribed herein with regards to certain materials and/or combination ofmaterials, this is not meant to be limiting and as such, other materialsand/or combination of materials having properties and/or characteristicssuitable to the desired end purpose may also be used. Moreover, itshould be further appreciated that although the method of the inventionis described herein with regards to certain processes and/or combinationof processes, this is not meant to be limiting and as such, otherprocesses and/or combination of processes suitable to the desired endpurpose may also be used. Moreover, unless specifically stated any useof the terms first, second, etc. do not denote any order or importance,but rather the terms first, second, etc. are used to distinguish oneelement from another.

I claim:
 1. A solar cell structure, comprising: A plurality of solarcells with at least two cells, a first solar cell, a second solar cell,wherein each of the solar cells include at least one of a p-nhomojunction and a p-n heterojunction, and wherein the solar cellstructure comprises a tunnel junction and a resonant tunneling structurebetween two adjacent solar cells, and wherein a first tunnel junctionand a first resonant tunneling structure are located between the firstsolar cell and the second solar cell, and wherein the first solar cellis comprised of first p-type layer and a second n-type layer, andwherein the first p-type layer and the second n-type layer are selectedone from single crystalline, poly-crystalline and nano-crystalline Si,and wherein first p-type layer serves as the substrate to support theplurality of cells, and wherein second solar cell comprises a thirdp-layer and a fourth n-layer, and wherein third p-layer is selected onefrom CdTe, ZnCdTe, CdGalnSe, and wherein the fourth n-layer is selectedfrom CdS, ZnCdS, CdGalnSe, and wherein said tunnel junction is comprisedof a n+-type layer and a p+-type layer, and the n+-type and p+-typelayers forming the first tunnel junction are selected one from Ge, Si,CdSeTe, CdGalnSe, and in one embodiment the tunnel junction is comprisedof, and the n+-type layer is disposed on top of second n-type layer offirst solar cell, and wherein p+-type layer of the first tunnel junctionis disposed with the first resonant tunneling structure, wherein thefirst resonant tunneling structure comprises of first quantum barrierlayer, a second quantum well layer, and a third quantum barrier layer,and wherein the first quantum barrier layer is selected one of ZnTe,ZnCdTe, ZnSTe, ZnSeTe, and the second quantum well layer is one selectedof ZnCdTe, CdTe, and the third quantum barrier layer is one selected ofZnTe, ZnCdTe, ZnSTe, and wherein second solar cell is disposed on thethird quantum barrier layer of the resonant tunneling structure, andwherein disposed on third quantum barrier layer is first p-type layer ofthe second solar cell, and the first p-type layer of second solar cellis disposed with a second n-type layer, the first p-type layer is oneselected from CdTe, ZnCdTe, and ZnSTe, and the second n-type layer ofsecond solar cell is one selected from CdS, CdInGaSe and CdZnS, andwherein the second n-type layer of second solar cell is disposed with an+-type facilitator layer facilitator layer, one selected from CdS,CdInGaSe and CdZnS, and wherein a top Ohmic contact is formed in theform of a grid, and wherein the first layer of p-Si serving as substrateis disposed with a bottom Ohmic contact.
 2. The solar cell structure ofclaim 1, further comprising two cells, a first solar cell and a secondsolar cell, wherein the first solar cell comprises at least two layers,and wherein the first layer of the first solar cell is selected one fromn-type CdS, n-CdInGaSe, and wherein the first n-type layer is disposedon a transparent thin film which serves as the top Ohmic contact, andwherein the transparent thin film is constructed of transparentconducting oxides such as one selected from tin oxide, doped tin oxide,indium tin oxide, doped indium tin oxide, and zinc oxide, and whereinthe top Ohmic contact is deposited on a substrate, and wherein thesubstrate is one selected from glass, quartz, and sapphire, and whereinthe second layer of first solar cell is selected one from p-type CdTe,p-ZnCdTe, and p-type CdInGaSe, and wherein a resonant tunnelingstructure and a tunnel junction (TJ) are disposed between the firstsolar cell and the second solar cell, and wherein the resonant tunnelingstructure comprises a first quantum barrier layer, a second quantum welllayer, and a third quantum barrier layer, and wherein the first quantumbarrier layer is one selected from ZnTe, ZnCdTe, ZnSTe, ZnSeTe, andwherein the second quantum well layer of resonant tunneling structure isone selected from Ge, ZnCdTe, CdTe, and the third quantum barrier layerof resonant tunneling structure is one selected from ZnTe, ZnCdTe, andZnSTe, and wherein the tunnel junction is comprised of a p+-type layerand an n+-type layer and wherein p+-type layer forming the tunneljunction is disposed on the third quantum barrier layer of the resonanttunneling structure, and wherein p+-type layer and n+-type layer formingthe tunnel junction are one selected from Ge, Si, CdSeTe, CdGalnSe, andwherein the n+-type layer of tunnel junction is disposed with the secondsolar cell, and wherein the second solar cell comprises a first n-layerand a second p-type layer, and wherein the first n-type layer and secondp-type layer are selected one from Ge and Si, and wherein second p-typelayer of second solar cell is disposed with an Ohmic contact facilitatorlayer, and the Ohmic contact facilitator layer is a p+-type layer, andthe facilitator layer is one selected from Ge, Si and GeSi, and whereina bottom Ohmic contact is formed on the Ohmic contact facilitator layer.3. The solar cell structure of claim 2, wherein the second solar cellcomprises a first n-type layer and a second p-type layer 2, and whereinthe first n-type layer is one selected from n-type amorphous Si, n-typeamorphous Ge, n-type polycrystalline Si, and n-type polycrystalline Ge,and the second p-type layer comprises more than one p-type interfacelayers and more than one quantum dot sub-layers, and wherein firstinterface layer is one selected from p-type amorphous Si, p-typeamorphous Ge, p-type polycrystalline Si, and p-type polycrystalline Ge,and wherein disposed on first p-type interface layer is a quantum dotsub-layer comprising of cladded quantum dot layers, and wherein each ofquantum dot layer comprises more than one cladded quantum dot layers,and wherein the cladded quantum dot layers are comprised of GeOxcladding and Ge nanodot core, and wherein the core of the quantum dot is2-6 nm in average diameter and the cladding thickness is in the range of0.5-1.5 nm, and wherein the assembly of quantum dots form a quantum dotsuperlattice (QDSL) layer, and wherein disposed on first quantum dotsub-layer is second p-type interface layer, which is selected one fromp-type amorphous Si, p-type amorphous Ge, p-type polycrystalline Si, andp-type polycrystalline Ge, and wherein disposed on second p-typeinterface layer is second quantum dot sub-layer, and wherein secondquantum dot sub-layer is comprised of cladded quantum dot layers, andwherein each of quantum dot layer comprises more than one claddedquantum dot layers, and wherein disposed on said second quantum dotsub-layer is a facilitator layer, and the facilitator layer is disposedwith bottom Ohmic contact, and wherein the facilitator layer is oneselected from p-type amorphous Si, p-type amorphous Ge, p-typepolycrystalline Si, and p-type polycrystalline Ge
 4. A solar cellstructure, comprising a plurality of solar cells: a first cell, a secondcell, a third cell and a fourth cell, and wherein each of the solarcells is at least partially constructed from a semiconductor materialhaving an energy band gap that harnesses photons having energies in apredetermined energy range responsive to the energy band gap, andwherein each of the solar cells includes at least one of a p-nhomojunction and n-p heterojunction, and wherein the solar cellstructure comprises a resonant tunneling structure and a tunnel junctionbetween first solar cell and second solar cell, and between second solarcell and third solar cell, and between third solar cell and fourth solarcell, and wherein the solar cell structure is disposed on a p-GaNsubstrate, and wherein p-GaN substrate is single crystalline anddisposed with one or more InGaN quantum wells, and wherein InGaN quantumwell layers are separated by GaN quantum barrier layers, and wherein ann-type GaN layer is disposed on the outer most GaN quantum barrierlayer, and wherein p-type GaN, InGaN quantum well and GaN quantumbarrier layers, and n-type GaN layer comprises the first solar cell onsapphire substrate, wherein disposed on n-type GaN layer of first solarcell is a nano-patterned region layer, and wherein nano-patterned regioncomprises masked regions and exposed regions, and wherein the maskedregions are one selected from SiO₂, Si₃N₄, SiO_(x)-cladded Si nanodotlayer, and wherein exposed regions expose the n-type GaN layer of firstsolar cell in the nano-patterned region layer, and wherein the exposedregions on n-type GaN layer is deposited with first buffer layerselected from one of n-Ge, n-Si, n-SiGe, n-ZnS or n-ZnMgS layer about2-10 nm in thickness, wherein masked regions in the nano-patternedregion layer has dimension in the range of about 20 nm×20 nm to about100 nm×100 nm on n-type GaN layer of first solar cell, and wherein thethickness of oxide masked regions in the nano-patterned layer is about10-100 nm, and wherein on first buffer layer, selected one from n-typeGe, Si, SiGe, ZnS, ZnMgS, in the exposed region of nano-patterned regionis deposited with a second buffer layer and a third buffer layer, andwherein second buffer layer and third buffer layers are one selectedfrom n-type doped Ge, Si, SiGe, CdS, ZnSSe, ZnSe, ZnMgTe, GaInP, CdTe,ZnCdSTe, and wherein the third buffer layer is nucleated on the secondbuffer layer and spreads over the said mask regions of thenano-patterned region layer, wherein the first tunnel junction and thefirst resonant tunneling structure are located between a first cell anda second cell, and wherein second solar cell is constructed from firstp-type layer and second n-type constructed from AlGaInP, and wherein inone embodiment the first tunnel junction is constructed from doped n+and p+ Ge layers, and the first resonant tunneling structure beinglocated between the p+Ge layer of first tunnel junction and a p-typelayer of AlGaInP of the second cell, and wherein said first resonanttunneling structure including a first quantum barrier layer, a secondquantum well layer, and a third quantum barrier layer are disposed onp+Ge layer of the first tunnel junction, and wherein first quantumbarrier layer is constructed from at least one of ZnSSe, AlGaInP,AlGaAs, and second quantum well layer is constructed from at least oneof AlGaInP, GaInP, GaAs, and third quantum barrier layer is constructedfrom at least one of AlGaInP, ZnSe, and wherein a second tunnel junctionand a second resonant tunneling structure are located between the secondsolar cell and the third cell, and wherein the third solar cell isconstructed from p-type and n-type GaInP layers forming a p-nhomojunction, and wherein the second tunnel junction is selected fromn+-p+ GaInP, n+-p+ GaAs layers forming second tunnel junction, andwherein p+ layer of tunnel junction is disposed with fourth quantumbarrier layer of second resonant tunneling structure, and wherein thesecond resonant tunneling structure includes a fourth quantum barrierlayer, a fifth quantum well layer, and a sixth quantum barrier layer,wherein the fourth quantum barrier layer is one selected form AlGaInP,AlGaAs, and ZnSSe the fifth quantum well layer is constructed from atleast one of GaAs, GaInP, and AlGaAs, and the sixth quantum barrierlayer is constructed from at least one of AlGaInP, ZnSe, and ZnSTe, andwherein a third tunnel junction and a third resonant tunneling structureis located between the third solar cell and the fourth solar cell,wherein fourth cell is constructed from n-type and p-type GaAs or n-typeand p-type GaInAs, and wherein the third resonant tunneling structureincludes a seventh quantum barrier layer, an eighth quantum well layer,and a ninth quantum barrier layer, wherein the seventh quantum barrierlayer is constructed from at least one of GaInP, AlGaAs, AlGaInP andwherein the eighth quantum well layer is constructed from at least oneof GaAs, GaInAs, and wherein the ninth quantum barrier layer isconstructed from at least one of GaInP, AlGaAs and AlGaInP, and whereinthe n-layer of the fourth cell has an n+ layer constructed from at leastone of GaAs, GaInAs to facilitate Ohmic contact, and wherein the firstsolar cell has an Ohmic contact on the p-GaN, and wherein the bottomOhmic contact is in the form of a grid permitting light through thesubstrate, and wherein the substrate is textured or deposited with adielectric coating which serves as an antireflection coating.
 5. Thesolar cell structure of claim 4, wherein the p-GaN substrate is replacedby a substrate one selected from sapphire, Si-on-sapphire, SiC, and Si,and wherein first p-GaN layer is disposed on a GaN buffer layer andwherein the GaN buffer layer is disposed on the said substrate, andwherein the top Ohmic contact is formed on first p-GaN layer, andwherein the top Ohmic contact is in the form of grid, and wherein thesubstrate is textured or deposited with antireflection coating.
 6. Thesolar cell structure of claim 5, wherein the second solar cell comprisesa first p-type layer and a second n-type layer, and wherein both ofthese layers are made of AlGaInP, and wherein the first p-type layercomprises at least three sub-layers including first sub-layer), secondsub-layer, and a third sub-layer, and wherein the second n-type layercomprises of three sub-layers comprising first sub-layer, a secondsub-layer, and a third sub-layer, and wherein the first p-type sublayeris constructed from AlGaInP such that it has an energy gap E_(g1), andwherein the second p-type sub-layer is constructed from AlGaInP with acomposition resulting in an energy gap of E_(g2), and the third p-typesub-layer is constructed from AlGaInP with an energy gap of E_(g1), andwherein the second p-type sub-layer has it energy gap E_(g2) higher thanenergy gap of sub-layer one and three (E_(g2)>E_(g1)), and wherein thethree n-type sub-layers comprising of first sub-layer, the secondsub-layer and the third sub-layer are constructed from AlGaInP and havetheir energy gaps E_(g1), E_(g2), and E_(g1), respectively, and whereinthe third solar cell constructed from GaInP is configured comprisingthree sublayers for the first p-type layer and three sub-layers for thesecond n-type layer, and wherein the second p-type sub-layer and secondn-type sub-layer are constructed from a higher energy gap (E_(g4)) GaInPthan the energy gap E_(g3) of first and third sub-layers.
 7. The solarcell structure of claim 4 comprising a plurality of solar cells, a firstsolar cell, a second solar cell, a third solar cell, a fourth solarcell, a fifth solar cell, and wherein each of the solar cells includesat least one of a p-n homojunction and n-p heterojunction, and whereinthe solar cell structure comprises a resonant tunneling structure and atunnel junction between first solar cell and second solar cell, andbetween second solar cell and third solar cell, and between third solarcell and fourth solar cell, and between fourth and the fifth solar cell,and wherein first solar cell comprises first p-type Si layer and asecond n-type Si layer, and said first p-type silicon layer serving as asubstrate which supports the said plurality of solar cell structure, andan n+ type layer is disposed on second n-type Si layer of first solarcell, and wherein first p-type Si layer, second n-type Si layer, andn+-type layer comprise of the first solar cell, and wherein disposed onn+-type Si layer of first solar cell is a nano-patterned region layer,comprising masked regions and exposed regions of said n+-type Si layer,and wherein masked regions are constructed from one selected from SiO₂,Si₃N₄, SiO_(x)-cladded Si nanodot layer, and wherein the exposed regionsexposes the n+-type Si layer of first solar cell in the nano-patternedregion layer, and the masked regions in the nano-patterned region layerhas x-y dimension in the range of ˜20 nm×20 nm to ˜100 nm×100 nm andthickness of oxide masked regions is in the range of ˜10-100 nm, andwherein the exposed regions of on n+-type Si layer are deposited withfirst buffer layer, selected one of n-ZnS, n-ZnMgS, n-ZnMgSSe layerabout 2-10 nm in thickness, and said first buffer layer is depositedwith a second buffer layer and a third buffer layer, and wherein thesecond buffer layer and third buffer layers are semiconductor layersselected from n-type doped layers of ZnSSe, ZnSe, ZnMgTe, GaAs, andGaInAs, and wherein the second buffer layer has a composition varyingfrom ZnS to ZnSSe, ZnBeS—ZnSSe, ZnMgSSe—ZnSe, ZnSeTe to ZnCdTe, orcombination of these, and the third buffer layer is disposed on thesecond buffer layer and spreads over on the mask regions of thenano-patterned region layer, and wherein the first buffer layer, secondbuffer layer, and third buffer layer comprises of the first resonanttunneling structure, and wherein first buffer layer serves as firstquantum barrier layer and second buffer layer serves as second quantumwell layer and the third buffer layer forms the third quantum barrierlayer and the third buffer layer is disposed with a fourth buffer layerwhich interfaces via the second buffer layer in transporting electronsfrom n+Si layer of the first solar cell, and wherein first resonanttunneling structure and the first tunnel junction are located between afirst solar cell and a second cell, and wherein the second solar cell isconstructed from first p-type layer and a second n-type layer, andwherein first layer and second layer of second solar cell are selectedone from GaInAs and GaAs and wherein the first tunnel junction isconstructed from an n+ type layer and a p+ type layer, and said n+ typelayer and p+ type layer forming the first tunnel junction are selectedone from GaInP, and GaAs, and wherein second p-type layer of secondsolar cell is disposed on p+ type layer of first tunnel junction, andwherein a second resonant tunneling structure and a second tunneljunction are located between the second solar cell and the third cell,and wherein the second resonant tunneling structure includes a fourthquantum barrier layer, a fifth quantum well layer, and a sixth quantumbarrier layer, and wherein the fourth quantum barrier layer is oneselected from AlGaInP, AlGaAs, and ZnSSe, and the fifth quantum welllayer is one selected from AlGaInP, GaInP, and AlGaAs, and the sixthquantum barrier layer is one selected from one of AlGaInP, ZnSe, andZnSTe, and wherein the second tunnel junction is comprised of an n+ typelayer and a p+ type layer, and wherein tunnel junction layers areselected one from GaInP and GaAs, and wherein the third solar cell isconstructed from first p-type layer and a second n-type layer, and saidlayers are constructed from GaInP forming a p-n junction, and whereinthe p+-type layer of second tunnel junction is disposed on sixth quantumbarrier layer of second resonant tunneling structure, and wherein athird resonant tunneling structure and a third tunnel junction arelocated between the third solar cell and the fourth solar cell, andwherein the third resonant tunneling structure comprises the seventhquantum barrier layer, the eighth quantum well layer, and a ninthquantum barrier layer, and wherein the seventh quantum barrier layer isconstructed from at least one of GaInP, AlGaAs, AlGaInP and the eighthquantum well layer is constructed from at least one of AlGaInP andGaInP, and the ninth quantum barrier layer is constructed from oneselected from AlGaAs and AlGaInP and wherein the third tunnel junctioncomprises an n+ type layer and a p+ type layer, and wherein the fourthcell comprises of a first p-type layer and a second n-type layer, andwherein the p-type layer and n-type layer of fourth solar cell areconstructed from AlGaInP, and wherein the fifth solar cell comprising afirst p-type layer and a second n-type layer, and the first p-type layerand the second n-type layer are constructed one selected from ZnSe andZnCdSSe, and wherein disposed between the fourth cell and the fifth cellis a fourth resonant tunneling structure and a fourth tunnelingjunction, and the fourth resonant tunneling structure comprises a tenthquantum barrier layer, an eleventh quantum well layer, and a twelfthquantum barrier layer, and wherein the tenth quantum barrier layer isone selected from ZnSSe, ZnMgSSe, AlGaInP, and eleventh quantum welllayer is one selected of ZnSe and AlGaInP, and twelfth quantum barrierlayer is constructed from ZnSSe, ZnMgSSe and the twelfth quantum barrierlayer is disposed with a fifth tunnel junction, comprising of a n+ layerand a p+ type layer, and the fifth tunnel junction layers are oneselected from ZnSe and AlGaInP and wherein the second n-type layer offifth solar cell is disposed with an n+ type Ohmic contact facilitatorlayer, and wherein the facilitator layer is selected one from ZnSe,AlGaInP, and wherein a top Ohmic contact layer is disposed, and the topOhmic contact layer is in the form of a grid which permits light, andwherein the first solar cell has a bottom Ohmic contact formed on thefirst p-Si layer serving as the substrate.
 8. The solar cell structureof claim 4, wherein the second buffer layer is a graded layer betweenthe first buffer layer and the third buffer layer in the nano-patternedregion, and wherein the second buffer layer has a composition varyingfrom Si to Ge, ZnMgS to ZnSSe, ZnSeTe to ZnCdTe, or combination ofthese.
 9. The solar cell structure of claim 6, which is biased in theforward biasing mode, and wherein the top Ohmic contact of fourth solarcell is negatively biased with respect to the bottom Ohmic contactdisposed on p-GaN layer, and wherein the solar structure emits light andfunctions as a light-emitting diode, and wherein the spectrum of lightis determined by the solar cell layers that are biased, and wherein theoperation of solar cell structure is selected depending on the imode ofoperation, and wherein the mode of operation is selected one of solarcell for light energy conversion into electricity and light-emittingdiode for emitting light, and wherein operation mode is controlled by amicroprocessor via switches and bidirectional converter, and whereinmicroprocessor determines the powering of load comprising one selectedfrom motor, display unit comprising of light-emitting devices, andcharging of a rechargeable battery.
 10. The solar cell structure ofclaim 6, wherein the structure is operated under concentrated sun lightconditions, and wherein the solar cell is adequately cooled using heatexchanger
 11. A solar cell structure comprising a plurality of solarcells, a first solar cell, a second solar cell. a third solar cell, afourth solar cell, a fifth solar cell, and wherein each of the solarcells include at least one of a homojunction and a heterojunction, andwherein the solar cell structure comprises at least one of resonanttunneling structure and a tunnel-junction, between the first solar celland the second solar cell, and second solar cell and the third solarcell, and third solar cell and the fourth solar cell, and fourth solarcell and the fifth solar cell, and wherein one or more solar cells haveat least two sub-cells, a bottom sub-cell and a top sub-cell, andwherein sub-cells are separated by a tunnel junction, and wherein eachsolar cell operates in a spectral regime determined by the energy gap ofthe photon absorber layer, and said solar cell structure is disposed ona substrate selected one from Ge, GaN, Si, and wherein the first solarcell is constructed of p-type Ge substrate and one or more layers ofn-type Ge, and the second solar cell is constructed one from GaAs andInGaAs, the third solar cell is constructed of GaInP, the fourth solarcell is constructed from AlGaInP, and the fifth solar cell isconstructed from ZnSe and ZnSSe, ZnCdSe, and wherein disposed betweenfirst solar cell and second solar cell is the first resonant tunnelingstructure, and the first tunneling resonant structure comprises firstquantum barrier layer, second quantum well layer, and third quantumbarrier layer, and wherein first quantum barrier layer is one selectedfrom GaInP, AlGaInP, AlGaAs, and second quantum well layer is oneselected of GaAs, GaInAs, and third quantum barrier layer is constructedfrom GaInP, AlGaInP, and AlGaAs and wherein first quantum barrier layeris disposed on n-type Ge layer, and the third quantum barrier layer isdisposed with first tunneling junction, and wherein the first tunnelingjunction comprises of an n+-type and a p+-type layer, and wherein then+- and p+-type layers are one selected one from GaAs and GaInAs, andwherein the second solar cell comprises of two solar sub-cells, a bottomsub-cell and a top sub-cell, and wherein the bottom sub-cell iscomprised of first p-GaAs layer and a thin first n-GaAs layer, andwherein a second tunnel junction separates the bottom sub-cell and topsub-cell, and wherein the second tunnel junction is comprised of ann+-type layer and a p+-type layer constructed from GaAs, and the topsolar sub-cell of second solar cell has a second p-GaAs layer and asecond n-GaAs layer, and wherein the second p-type GaAs layer is thinnerthan the first p-GaAs layer of bottom sub-cell, and wherein disposed inbetween the second solar cell and the third solar cell is the secondresonant tunneling structure, and wherein the second tunneling resonantstructure comprises a fourth quantum barrier layer, a fifth quantum welllayer, and a sixth quantum barrier layer, and wherein the fourth quantumbarrier layer is disposed on second n-type layer of top sub-cell ofsecond solar cell, and the fourth quantum barrier layer is one selectedfrom AlGaInP, AlGaAs, and the fifth quantum well layer is one selectedfrom GaInP and GaAs, and sixth quantum barrier layer is constructed fromAlGaInP and AlGaAs, and where in disposed on the sixth quantum barrierlayer of second resonant tunneling structure is the third tunnelingjunction, and the third tunnel junction comprises a n+ type layer and ap+ type layer, and wherein n+ and p+ type layers are one selected fromGaAs and GaInP, and wherein p+-type layer of third tunnel junction isdisposed with the third solar cell, and wherein the third solar cell isconstructed from GaInP and has a bottom sub-cell and a top sub-cell, andwherein the bottom sub-cell has a first p-type GaInP layer and firstn-type GaInP layer, and the top sub-cell of the third solar cell has asecond p-type layer and a second n-type layer of GaInP, and wherein thesecond p-type GaInP layer is thinner than the first p-type GaInP layer,and a fourth tunnel junction separates the bottom sub-cell and topsub-cell of third solar cell, and wherein the fourth tunnel junction iscomprised of an n+-type layer and a p+-type layer, and disposed on thesecond n-type layer of the third solar cell is the third resonant tunnelstructure, and wherein the third resonant tunneling structure comprisesof seventh quantum barrier layer, an eighth quantum well layer, and aninth quantum barrier layer, and wherein the seventh quantum barrierlayer is one selected from AlGaInP and ZnSe, and the eighth quantum welllayer is one selected from AlGaInP and GaInP, and the ninth quantumbarrier layer is constructed from AlGaInP and ZnSe, and disposed on theninth quantum barrier layer is the fifth tunnel junction which comprisesa n+-type layer and on top of which is a p+-type layer, and whereinthese layers are one selected from AlGaInP and GaInP, and the fourthsolar cell is disposed on p+-type layer of the fifth tunneling junction,and the fourth solar cell is constructed from AlGaInP, and the fourthsolar cell comprises a bottom sub-cell and a top sub-cell, and thebottom sub-cell has a first p-type AlGaInP layer and first n-AlGaInPlayer, and a second p-type AlGaInP layer and a second n-type AlGaInPlayer forming the top sub-cell of the fourth solar cell, and wherein thefirst p-type AlGaInP layer is thinner than the second p-type AlGaInPlayer, and wherein a sixth tunnel junction separates the bottom sub-celland top sub-cell, and the sixth tunnel junction is comprised of ann+-type layer and a p+-type layer constructed from AlGaInP, and disposedon the second n-type layer of fourth solar cell 5045 is the fourthresonant tunneling structure, and wherein the fourth resonant tunnelstructure comprise of tenth quantum barrier layer, eleventh quantum welllayer, and twelfth quantum barrier layer. And wherein the tenth quantumbarrier layer is one selected from ZnSSe, ZnMgSSe, AlGaInP, and eleventhquantum well layer is one selected of ZnSe and AlGaInP, and twelfthquantum barrier layer is constructed from ZnSSe, ZnMgSSe and the twelfthquantum barrier layer is disposed with seventh tunnel junction,comprising of a n+ layer and a p+ type layer, and the seventh tunneljunction layers are one selected from ZnSe and AlGaInP, and wherein thefifth solar cell is disposed on the p+-type layer of seventh tunneljunction, and the fifth solar cell comprises a bottom sub-cell and a topsub-cell, and the bottom sub-cell has a first p-ZnSe layer and a firstn-ZnSe layer, and wherein the top sub-cell comprises of a second p-ZnSelayer and a second n-ZnSe layer, and wherein the first p-ZnSe layer ofbottom sub-cell is thicker than second p-ZnSe layer of top sub-cell, andwherein the eighth tunnel junction separates the bottom sub-cell and topsub-cell, and the eighth tunnel junction is comprised of an n+-typelayer and a p+-type layer, and eighth tunnel junction is constructedfrom one selected from ZnSe and AlGaInP, and the second n-type layer offifth solar cell 5061 has an n+-type layer facilitating the formation oftop Ohmic contact grid on second n-type layer, and wherein the bottomOhmic contact is formed on p-Ge substrate.
 12. The solar cell structureof claim 11, which is biased in the forward biasing mode, wherein thetop contact of fourth solar cell is negatively biased with respect tothe bottom Ohmic contact on p-Si substrate, and wherein the solarstructure emits while light, and functions as a light-emitting diode,and, wherein the operation of solar cell structure is selected biasingit to a mode of operation, and wherein the mode of operation is selectedone of solar cell for light energy conversion into electricity andlight-emitting diode for emitting light, and wherein operation mode iscontrolled by a microcontroller, and wherein microcontroller determinesthe powering of load such as a motor via an electrical interface,display comprising of light-emitting devices, and charging of arechargeable battery.
 13. The solar cell structure of claim 12, which isbiased in the forward biasing mode, wherein the top contact of fourthsolar cell is negatively biased with respect to the bottom contact onp-Ge substrate and wherein the solar structure is operated as a lightemitting diode structure, and wherein the spectral distribution oflight-emitting diode structure is determined by how many of the solarcells are powered, and wherein the selection of solar cell andlight-emitting diode operation is controlled by a microcontroller, andwherein microcontroller determines the powering of load, charging of arechargeable battery, biasing the solar cell structure as light-emittingdiodes, depending on the application.
 14. The solar cell structure ofclaim 13, wherein the structure is operated under concentrated sun lightconditions, and wherein the solar cell is adequately cooled using heatexchanger.